When an author or creator takes two previously unrelated works and puts them into a single, shared continuity.
No, it's not about welding two weapons together to make a double barrel cannon. That would be cannon welding, and that would be awesome.
Sci-fi and fantasy authors don't always write all their novels in the same continuity. A budding new author's first published book might be about space pirates in the 27th century, while his sophomore effort might instead be about 21st century scientists reverse-engineering a flying saucer. In response to popular demand, he might end up writing a sequel to one, or even both of these novels. Flash forward about 20 years — the author has grown wealthy from writing stories about Captain Flash Orangebeard and Dr. Smith of Mars, but he's running out of ideas and the two long-running series are in danger of getting stale. What does he do to keep the public's interest, and breathe new life into the storylines?
Many long-lived genre authors tend to resort to Canon Welding, usually at a later point in their career. They combine two or more distinct series they've created into a single continuity. This isn't just a one-off Crossover; for series with radically different premises, the foundations of one or both stories can be altered forever.
By combining the two series together, the author can introduce fans of one series to characters they may not be familiar with, inducing them to go out and buy the works in that series, and hopefully attract high sales from fans of both storylines. When done well, it can add a more epic feel to the tale, explore aspects of the two storylines not previously delved into, and make lots of money for the author and his publisher (and there are many examples of this, perhaps most famously Lord of the Rings). When done poorly, especially with stories with radically different settings or styles, it looks and feels like a shallow money-grab and can potentially be a shark-jumping moment for both series.
Modular Franchise is when it's done at a corporate level. Compare Shared Universe, which can be created through Canon Welding if it wasn't shared from the beginning.
This was very nearly canon, as it happens; the invasion force from another galaxy from the finale of Season 2 of Blake's 7 were intended to be the Daleks at one point, but for one reason or another the idea was dropped.
Also, one of the canonical Buffy the Vampire Slayer comics has a cameo appearance of the Tenth Doctor and Rose. And a character named "William the bloody awful poet" (i.e. Spike when he was still human) appears in the Eighth Doctor Adventures novel Camera Obscura. (Note that Doctor Who is still a TV series in the Buffyverse, and that neither of these represents a collaborative effort on the part of the creators or writers of either franchise)
Douglas Adams' character Professor Chronotis from the novel Dirk Gently's Holistic Detective Agency originally came from Adams' TV script Shada (which was only partly filmed due to an electricians' strike). In the Dirk Gently novel, it's not spelled out that Chronotis is a Time Lord in hiding, for copyright reasons, but it's clearly meant to be the same character. There's even an oblique little reference at the end that a strange young man came and permanently disabled his time machine, while the POV character wasn't around...
Super Robot Wars is already a Crossover series, with nearly as many canons as it has games - most entries take place in their own continuities, created by fusing together the stories of whichever mecha shows are featured in that particular game. But there are a few characters who show up in multiple continuities, and while most of them are Alternate Universe versions of each other, Gilliam Yeager, whose gimmick involves hopping between universes, has been implied to be the same person in all his appearances, no matter what continuity you're in. Which in turn means that any games with Gilliam in them would be part of the same multiverse.
ZO and J fight a multi-seasonal batch of monsters in Kamen Rider World (8-minute theme park thingy, may not be canon but never said not to be, and not contradicting anything) which puts all three hiatus movies (yes, Shin provided a monster) into old-school KR continuity. Kuuga's mention of a Professor Hongo (and an imitation of him, which means he must have known the Hongo) put Kuuga and Agito into it as well. However, Decade makes the multiverse more complicated with its alternate universes bearing variable resemblence to - and rarely literally being - the worlds of the actual series it's crossing over with. We even get Black and Black RX as separate worlds, as well as Kuuga and Agito, with alternate versions of some of the same people.
Even moreso, late in Double a member of Foundation X can be seen looking over data on OOO's Core Medals. Nothing came of this for over a year, until the Fourze/OOO crossover Movie Wars Megamax revealed that Foundation X would be playing a role, this time using the Astro Switches from Fourze...and that Double and the first seven Showa Riders would be teaming up with Fourze and OOO. Given how Astro Switches pretty much are Gaia Memories this makes some sense (except for the Last One thing).
Some worlds have versions of Riders of other worlds with no dimension-hopping. For example, Dark Kabuto, Dark Kiva, Ryuga, and Orga live in a world where monsters rule, and have no connection to Kabuto, Kiva, Ryuki, or Faiz. It's the second Ryuga we meet, and no, the first wasn't in the World of Ryuki, either.) It also means Double and OOO take place in the World of the Rider War, as Double does no dimension hopping to meet Decade, and OOO does no dimension hopping to meet Double. (That last point, though, is pretty much complicated by the sheer canon inconsistencies between Kamen Rider OOO and Movie Wars Core.)
Decade's own Mind Screw-itiude and A Wizard Did It attitude makes it nigh useless for working out continuity issues or finally answering which of your favorite Riders can kick the other's ass. Post-Decade teamup occasions not requiring any dimension-hopping (as it was with pre-Decade teamup occasions) would seem to have all things Kamen Rider in one universe, with past Riders still out there after they leave our sight (like any character in any show who has been Put on a Bus.) It would seem that none of the AR Worlds were the one universe KR usually takes place in.
However, don't put away your migraine medicine just yet: OOO, Den-O, All Riders: Let's Go, Kamen Rider would have the Decade World of Kuuga Big Bad instead of the Kamen Rider Kuuga Big Bad representing the Grongi in the Legion of Doom, which would have welded Decade and its madness right back in...if it weren't for the fact that the movie's ending implicitly makes the whole ordeal non-canon to everything by virtue of settling on a Close Enough Timeline where, in regards to Kamen Rider OOO, Hina doesn't know Eiji. (Also, every character who appeared in Decade had their updated design from that series instead of their original designs, but that can be ignored - you'd use the shiny Decade suits instead of making all new ones that don't look as good in order to perfectly match the 1970s versions.)
It's gotten to the point that, near as anyone can tell of the yet-unreleased Kamen Rider X Super Sentai Superhero Taisen movie, nobody except MAYBE Marvelous and Decade know what the hell is going on. Even the narrator is baffled.
And with Compati Hero Series The Great Battle IV this would tie in Gundam, the Ultra Series, and Kamen Rider, et al. with the aforementioned Super Robot Wars and...well. Just let your imagination go wild, one supposes.
That's not even bringing the fact that since Marvel Comics has a huge multiverse there's every universe in that they can Crossover with and therefore due to Marvel Zombies vs Army of Darkness, Ash Williams could very logically have a meeting with the Doctor if they slipped through into the other's universe. This means that we could logically have Ash accidentally awaken Deadites in multiple time periods and have the Doctor and Ash work together to stop them in all of them.
In the mid-2000s, writer Simon Furman ruled that every single Transformerscontinuity forms part of a massive multiverse of different timelines, dimensions and universes, and sometimes featured Crossovers in his stories (for example, the 'Generation One' Galvatron and several others making a cameo appearance in a Transformers Armada comic). He also ruled that Unicron and Primus are constant forces in this multiverse, and though they can be destroyed in one reality their consciousness lives on in another. Curiously, his next range of comics for IDW seemed to separate from this idea altogether.
It gets better. Courtesy of Axiom Nexus, any Transformers series can interact with any other.
Even better, the Transformers franchise itself was an amalgamation of several unrelated lines of Japanese die-cast toys (Jetfire/Skyfire was a VF-1 Valkyrie), with most of the welding done by the fine folks at Marvel Comics.
The OVA Giant Robo: The Day The Earth Stood Still and its companion manga The Day The Earth Burned incorporates practically all of Mitsuteru Yokoyama's work, including the first magical girl Sally the Witch, the tokusatsu show Iga No Kagemaru, the eponymous giant robots, and historical characters from both the Water Margin and Romance of the Three Kingdoms.
Once Matt Tracker figure was released as Specialist Tracker in one of G.I. Joe toy series, M.A.S.K. has been adopted into G.I.Joe Universe.
Witch Girls Adventures is a 'verse created almost entirely through Canon Welding. The 'verse started as a fetish e-zine called "The Shrinking Sorceress" by MANGA GRAPHIX, dedicated to sorceresses transforming people into animals and inanimate objects. Later on, many of the same people went on to write Witch Girls Tales, theoretically a comic about young witches getting into mischief with their powers, and several characters and concepts from MANGA GRAPHIX stories ended up in the new 'verse. Completely independently, a different author wrote a comic called "Princess Lucinda," about the titular princess' love for wickedness and transforming people over the slightest offense. The Witch Girls Adventures game was created as a team-up between Channel M (the reconstituted MANGA GRAPHIX) and Abby Soto (the creator of Princess Lucinda), using characters from "The Shrinking Sorceress" (including some that hadn't yet appeared in Tales), Witch Girls Tales, and Princess Lucinda all in a single standalone universe.
The Soul Series is also seem to take place in the distant past of the Tekken universe due the presence of Yoshimitsu in every installment of both series.
Video game Blair Witchproject I: Rustin Parr sets the original Blair Witch movie and video game Nocturne, made by the same creators in one world.
And apparently the first BloodRayne game implies several times that it's set in the same world as Nocturne.
There is a crossover between Wonder Woman and Fafhrd and the Gray Mouser. The latter's world was adapted into Dungeons & Dragons setting. Of course, there was a fair amounts of retcons in The DCU and revised editions of D&D, but it's quite possible that the link estabilishing connection between the three still exist in some form.
Thanks to the d20 edition of Call of Cthulhu you can play as Dungeons and Dragons characters in Call of Cthulhu and introduce the Cthulhu Mythos into Dungeons and Dragons. It however doesn't stop there as the Call of Cthulhu sourcebook known as The Malleus Monstrorum not only mentions every major Mythos entity, the book also manages to throw in The Thing (1982), The Martians of The War of the Worlds, and The Wicker Man and several of Stephen King's characters as Nyralathoep's avatars.
Shin Mazinger is quickly reaching a critical level of this, with a woman from Violence Jack turning out to be Kouji Kabuto's mother.
Violence Jack has incorporated Devilman and later Devilman Lady as taking place in one Universe that resets itself and all main characters are really incarnations of Akira Fudou. And because the series is also Deconstructor Fleet for all other Go Nagai's manga, there are many theories incorporating them into it in all incarnations, which is possible thanks to the nature of this world. Cameos and crossovers between his works are so often it's pretty easy.
And now the latest example of this is Devilman Grimoire, where Jun Fudo and Aoi Kurosaki from Devilman Lady are shown to be teachers at Akira and Miki's school. They are also lovers. Alphonse and Himura, from the 1970s Devilman anime series, also feature.
On the other hand, Magic Kaito is definitively in the same universe of Yaiba!; the characters went to the same school called Ekoda, and the Detective Conan OVA Conan vs Kaitou Kid vs Yaiba was originally a Magic Kaito story arc (and not All Just a Dream), in which Kaito attempts to steal a magic sword, just before he found out what he was meant to be going after. Not to say, Aoko's gossip mill friend Keiko's "very reliable source" is Sayaka, the main girl in Yaiba!.
The mangaka group CLAMP has been known for self-crossovers for many years, but their twin series Tsubasa Reservoir Chronicle and Xxx HO Li C are meant to tie all their works—both present-day and fantasy—into a single continuity.
The second All Stars-movie features 17 magical girls from 5 different continuities. From the previews it seems to feature some of the different baddies, too.
All Stars DX 3 ups the number to 21 from 6 continuities and the brand-new New Stage brings it to a grand total of 28 from 7. It overlaps with Remember the New Guy as a lot of Cures that show up in one movie weren't in the movie before that.
All Stars New Stage delivers a Retcon of sorts now stating that 23 magical girls from 6 different continuities saved the day. Then 5 more from the 7th hopped in and it was madness.
Interestingly, it seems that Smile Pretty Cure is attempting a bit of a Canon Weld experiment themselves - an episode midway through reveals that Yayoi's mother works for Fairy Drop, the store owned by Erika Kurumi's mother. And said mother is mentioned, at least by last name.
Eiichiro Oda re-used Ryuuma, a character from his one-shot manga Monsters, as a (zombified) villain in One Piece and his home country was mentioned to be part of the New World (the second half of the Grand Line). He later confirmed that Monsters was incorporated into the backstory of the setting.
Mahou Sensei Negima! reveals that all of Ken Akamatsu's major works exist in the same universe. The ties between Negima and Love Hina are obvious with Setsuna being a Shinmeiryuu swordswoman, which is lead by the Aoyama family from Love Hina; the reference to A.I. Love You is found in a single panel, although it's kinda important, as the protagonist of that series is implied to have written the code that enables Chachamaru to have a soul.
Before he gave the world Dragon Ball, Akira Toriyama's first popular series was a comedy called Dr. Slump, about a robot girl and the slob scientist who created her causing havoc in a weird place called Penguin Village. About a year into Dragon Ball, Toriyama had Goku visit Penguin Village and meet most of the Slump cast, thus joining the two series into one universe.
This was mostly done as an attempt to use Dr. Slump's popularity to help increase readership of Dragon Ball, as it wasn't the huge hit it would eventually become yet. In contrast, the crossover has had the opposite effect in later years: many fans, especially outside of Japan, only know the Dr. Slump cast because of their guest spot on Dragon Ball.
It's gotten to the point that Arale's made it into at least three Dragon Ball video games as a playable character! Budokai Tenkaichi 3, Origins and Revenge of King Piccolo, to be precise. The first one also caused a good amount of rejoicing for those who knew her.
Endings of Getter Robo Armageddon and New Getter Robo in which Armageddon versions of Ryoma, Hayato and Benkei and New version of Ryoma ends in Warrior Heaven, alongside countless Getters, fighting unknown monsters has hinted that all Getter's separated continuities (two mentioned above, Ken Ishikawa's manga continuity, Getter Robo DASH manga and anime Getter Robo Go and Shin Getter Robo Vs Neo Getter Robo) might exist in the same Multiverse.
Several years ago, Hidenori Kusaka and Satoshi Yamamoto worked on a short Pokémon Ranger manga that was only released online. As it turns out in the Platinum arc of Pokémon Special, the events that occured in that online comic are indeed canon.
The DC and Marvel universes were born from this trope; originally, the titles published by each company did not overlap, but over time, cameos, Crossovers, and inside references combined to form the comic books into one big, interconnected web. That's not even counting the Amalgam universe.
Mind, Marvel started this with the first issue of Spider-Man. And even before that, Marvel started this 21 years earlier in the Timely Comics era, when Human Torch faced off against Namor the Submariner for the first time. They teamed a few more times over the next few years, and some of the less prominent characters occasionally got involved. Then, in 1946, Timely launched the All-Winners Squad, teaming up existing characters like Captain America, the Human Torch, and the Sub-Mariner (among others)
DC started it twenty years earlier in All-Star Comics #3 with the Justice Society's first meeting. To this day, it's generally accepted that the Justice Society is the first-ever example of a super hero team lasting longer than a single issue in comics history.
Image Comics is an interesting case. Originally, all of its titles took place in a shared universe. Over time, the original Image partners focused on their own corners of the Image Universe, causing the continuity to split into several distinct sub-continuities. The Shattered Image crossover made the split official. But Image partners still occasionally "borrowed" each others' characters, so the sub-universes still interacted. As new, non-partner creators become more prominent in Image Comics, they started building universes of their own, and they occasionally used the Image partners' characters. For example:
Characters from Jay Faerber's creator-owned series (Noble Causes, Venture, Firebirds, & Dynamo 5) appear in each other's books all the time, creating a loose-knit "Faerberverse".
Robert Kirkman's characters occasionally cross over in a similar fashion (and some times become supporting cast - especially in Invincible).
The Kirkmanverse and Faerberverse intersect at a number of points, especially The Pact mini-series. Other Image characters, such as Savage Dragon and Shadowhawk, often pop up. So far, however, Spawn had yet to make an appearance...
...Until the recently announced Image United, which brings together characters of all of the current Image partners (Erik Larsen, Rob Liefeld, Todd McFarlane, Marc Silvestri, Robert Kirkman, and Jim Valentino), as well as Whilce Portacio and several other creators.
Angela from the Spawn universe jumped ship to the Marvel Universe at the close of Age Of Ultron. Though it's made clear that her home dimension exists outside the Marvel U.
Eclipse Comics' four-part crossover mini-series, Total Eclipse brought together virtually all company-owned and creator-owned characters that the company published.
The Harlem Heroes strip in 2000 AD (about a basketball team with jetpacks in 2050) appeared to be totally unconnected to the 22nd centuy of the Judge Dredd universe until the son of one of the Heroes (John "Giant" Clay) joined the Judges (as Judge Giant). The Judge Dredd story "Hammerstein" suggested ABC Warriors was also set in the past of the Dreddverse, but later ABC Warriors stories contradicted this.
Dredd has also had crossovers with other 2000 AD strips whenever the writers felt like it, most notably Strontium Dog and the story Helter Skelter (where Garth Ennis basically crossed ALL his favourite strips over with Dredd).
Meanwhile, 2000 AD stalwart Pat Mills has crossed over everything he's ever written for 2000 AD with each other. Invasion!/Savage, Flesh, Ro-Busters, ABC Warriors, and Nemesis the Warlock all slot together.
Ian Edginton does the same thing with his 2000 AD strips: both Stickleback and The Red Seas share a secret organisation, little mentions and character cameos abound, and the same brand of monster appears in Stickleback, Ampney Crucis Investigates, and Detonator X.
Even more Edginton crossovers: Sir William Ashbless, immortal designer of the titular ship in Leviathan made a cameo appearance in Stickleback and his shipping company, White Hart Line, got name dropped in Ampney Crucis Investigates. Also a few locations have been repeated across the various strips at different periods in history.
John Smith did a similar thing from the start in order to make his stories stand out: all his initial Future Shocks linked in to an organisation called Indigo Prime, and a couple of Indigo Prime agents also appeared in Tyranny Rex. Indigo Prime then got its own series, and eventually crossed over with Smith's DC Comics series, Scarab.
Alan Moore, as time has gone on, has turned League of Extraordinary Gentlemen into this, making vague references to the source material for Ozymandias and The Black Freighter. Oh, sure, it's only references to the inspirations for them, and Moore would probably rather have his skin boiled than actually go further than that, but this is Alan Moore, there are no coincidences.
Even ealier Millar estabilished connections between three comics published by different companies - Wanted, Chosen and The Unfunnies. The reason why at the end of the Chosen media doesn't report Antichrist's miracles is that they're controlled by supervillains from Wanted. And Troy Hicks from Unfunnies helped Satan rape Antichrist. Never published Run! was supposed to be set in that world too.
Of course, Grant Morrison is one of the architects of Hypertime (the other being Mark Waid) which posits that it is all true. Under this concept, the events of DC vs. Marvel exists somewhere in continuity.
Almost all of Morrison's DC works are tied to each other, as well to the real world, forming a big "Morrisonverse". Here's how it goes: In All-Star Superman Superman creates the infant universe Qwewq. In JLA we see the heroes discover (a version of) Qwewq. Both in ASS and in JLA: Confidential we see that Qwewq actually contains "our" Earth, i.e. a realistic Earth with no superheroes. The final Morrison-penned issues of Doom Patrol and Animal Man take place in a realistic world with no superheroes (and they both share the same colour scheme, meaning it's the same world in both), which is presumably Qwewq, i.e. "our" world. In Seven Soldiers we find out the ultimate fate of Qwewq (or at least one version of it). Final Crisis (which takes place in the same universe as JLA) refers to Bleed (the "sea" that separates different universes in the DC multiverse) as "ultramenstruum", and the same term is used is The Invisibles, implying that the Invisibles universe is a part of the larger DC multiverse. If we accept that Qwewq is "our" universe, this means our universe exists inside a larger universe populated by superheroes. Both Flex Mentallo and The Filth feature the "real" world to which superheroes from outside this world burst in; thus, the real world in both these comics could be (a version of) Qwewq. And then a huge chunk of the Damian Wayne stories written by Morrison that take place in the not too distant future were revealed to be set in the past of DC One Million and ends with Damian training Terry McGinnis from Batman Beyond to become his successor. Lastly it also seems that DC One Million takes place in the future of All Star Superman as Solaris and Kal Kent appear and happens to be the story of how Superman ended up having to fix the sun. To sum it up, almost all of Morrison's major works for DC are welded together, though admittedly some of the links between them are vague.
When Semic Comics, a French comic publisher, decided to revive the characters it inherited from defunct Editions Lug, editor Jean-Marc Lofficier set out to link over 2000 largely unrelated characters from just about every comic book genre into a single continuity. Some characters had to be revamped fairly drastically to fit in, and a few had to be revamped to avoid duplication.
In Non Sequitur, Wiley frequently used four separate sets of recurring, originally nameless, characters: a silent Everyman who'd observe some of the comics' less absurdist strips, a Leisure Suit Larry-ish barfly, a snarky Bratty Half-Pint girl and the Sunday-only diner owner "Offshore" Flo (and her tall-tale telling patron, Eddie). Gradually, the characters started interacting: the Everyman and the barfly were seen hanging out at the bar, the girl and Flo would occasionally be seen reacting to something from the Everyman's radio talk show. Eventually, Wiley brought all these elements together to form a central cor: Joe (the everyman) and Bob (the barfly) are brothers, Danae (the little girl) and her little sister are Joe's daughters and Flo is Joe and Bob's mother. Eddie remains "just" Eddie.
Present in horror films long, long before Freddy vs. Jason. In the Universal Horror series, Frankenstein Meets the Wolf Man triggered the tendency to pile on the monsters, insisting that Dracula, the Wolf Man and Frankenstein's Monster all existed in a common universe. This does not work well with continuity (The Wolf Man (1941) takes place in the present day while the others happen in a dimly-characterized past), but they didn't care much by that point (Universal's horror films of the 40s are strikingly dumber and more juvenile than those of the 30s).
Quentin Tarantino has created a largely common universe of his films by including subtle crossreferences (for instance, characters commonly refer to others; Mr. White mentions Alabama and Mr. Blonde has Scagnetti as a parole officer, Vic Vega and Vincent Vega are brothers, etc.) and cameos, but he says that his movies are divided into two universes.
Tarantino has also stated that Django and his wife Broomhilda are intended to be the ancestors of John Shaft.
In all honesty Alien and Predator were never really meant to part of the same universe. The Xenomorph head seen near the end of Predator 2 was only meant as a joke but inspired dozens of video games and comic books until the connection was made completely canon in the Alien vs. Predator movies. Hints placed by Ridley Scott in Prometheus and in the latest re-edit of Blade Runner suggest that they (and of course the rest of the Alien franchise, and transitively Predator) share the same continuity.
Before Final Wars, in Godzilla, Mothra, and King Ghidorah: Giant Monsters All-Out Attack (aka GMK), there's another acknowledgment of the US Godzilla that works even better at integrating it into canon. In this film's continuity, Godzilla hasn't been seen since the original 1954 attack on Tokyo, and many people no longer remember what he looked like. It's said that there was a reported Godzilla attack in New York a few years earlier, but it's unconfirmed whether it was the real Godzilla. This fits perfectly with the US film, wherein Japanese characters identify the creature as "Gojira." Indeed, GMK has characters mistaking another monster (Baragon) for Godzilla, which may be an intentional retcon for how the creature in the US film got misidentified that way.
Fantasy author Michael Moorcock gradually connected almost every single character he'd created into a Myth Arc revolving around the concept of the Eternal Champion. Indeed, this article was originally titled "The Moorcock Effect" in reference to him.
Given that his Doctor Who novel The Coming of the Terraphiles features a Captain Cornelius, who may or may not be another aspect of the Eternal Champion (much like Jerry Cornelius) that probably ties the Eternal Champion into the Whoniverse as well! There's also a "Second Aether", which, spelling aside, ties the Whoniverse to the Second Ether sequence.
Robin Hood went through several rounds of this, along with Adaptation Displacement. Maid Marion, Friar Tuck, and Alan-a-Dale were all characters from separate folk tales, and it was only later that Robin Hood stories had anything to do with Richard the Lionheart or Prince John.
In the 1980s, Isaac Asimov wrote a series of novels that linked his Robots, Empire, and Foundation novels into a single continuity. He also even went so far as to in Foundation and Earth suggest (via throwaway reference in dialogue) a tenuous connection between the Robots/Empire/Foundation series and an otherwise seemingly unrelated, comparatively obscure time-travel novel, The End of Eternity, which he wrote in the 1950s (interestingly enough, if they were in the same continuity, the end of the novel would effectively have caused the entirety of the events in the rest of the series, because said ending revolves partly around allowing humanity to expand into space instead of mouldering on Earth).
Don't forget throwing in a reference to his standalone novel Nemesis in one of the later Foundation books, despite the fact that Nemesis and the Robots/Empire/Foundation books taking place in the same universe makes no sense whatsoever (not even the space-travel physics work the same way). Though it may have been a literal reference to his novel.
The officially licensed Fan Sequel, Psychohistorical Crisisretcons a number of aspects of the Foundation series, and also ties in Asimov's otherwise unrelated short story Nightfall.
Actually, they were tied together much before that, notably by the Talamasca (introduced in Queen of the Damned and later a key player in both the vampires and witches novels) and a few common supporting characters like Aaron Lightner. In other words, the Witches novels avowedly take place in the same world as the Vampire Chronicles from day one, though their interactions increase substantially over time. Hints in The Vampire Lestat also indicate that Rice's least-liked novel, The Mummy, also shares a continuity with these series.
The novel The Queen of the Damned establishes that witches and spirits are real. Memnoch the Devil claims that God, the angels, and The Devil are all real.
The Hobbit was not, at the time of its writing, intended to be in the same continuity as The Silmarillion, which Tolkien regarded mainly as personal recreation and had as yet no intent of publishing. Despite this, he couldn't help throwing in a few names and locations that referenced The Silmarillion. When he began writing the sequel that would become The Lord of the Rings, he went whole-hog and moved The Hobbit to Middle-Earth, The Silmarillion becoming the Back Story of the novels. In fact, the ring that Bilbo found was originally just an ordinary, harmless magic ring and nothing more, and Gollum, having no motive to kill Bilbo, happily led him to safety at the conclusion of the riddle game. It wasn't until The Lord of the Rings was being written that Tolkien decided that it was the ring, and he altered the Bilbo/Gollum encounter to be a little more sinister. The in-universe explanation for the altered narrative is that Bilbo wrote the first version while under the influence of the ring as he wanted to conceal the actual circumstances of his acquiring it, while the revised, true version was written later, after he was no longer in the ring's grasp.
Tom Bombadil, Goldberry and Old Man Willow originally appeared in a poem published in 1933. They had no connection to Middle-Earth until the writing of The Lord of the Rings was in progress, and that didn't turn them into anything more significant than a Wacky Wayside Tribe.
Robert A. Heinlein did this towards the end of his career, incorporating all his previous stories (often with radically different universes) into one meta-universe, thanks to a handy trans-dimensional device invented by one of his characters. Then he brought the John Carter of Mars series in, and the Oz books, and eventually all fiction ever created.
Though he did give preference to the ones he liked, and especially those written by authors with whom he was personally acquainted; one of the transdimensional 'jumps' involved taking the characters into the Lensman universe created by his friend, E. E. “Doc” Smith.
Nearly all main characters he ever wrote are in one scene at the end of The Cat Who Walks Through Walls. It involves most of them trying to recover Mycroft Holmes, whose death was perhaps the biggest Tear Jerker he ever wrote. Towards the end the characters are aware they are in a story, and find the Author to be a bastard...
Larry Niven originally had two continuities: the first was the "slowboat" stories of early colonization of space by humanity (featuring the novels "World of Ptaavs", the Gil Hamilton stories, and "A Gift From Earth"), while the second featured faster-than-light travel and aliens (featuring the stories of Beowulf Shaeffer, Louis Wu, and the Ringworld. And then he wrote his short story "Relic of Empire", which combined the two continuities and created the Known Space universe.
Edgar Rice Burroughs did this when Tarzan first traveled to the underground world of Pellucidar to rescue that title's hero. It grew from there under his pen and under the pen of others using his characters.
There was a series of action figures: "Tarzan on Mars". Of course, poor Edgar had nothing to do with it.
In A Fighting Man of Mars, Jason Gridley appears. Since Gridley met Tarzan in Tarzan At the Earth's Core, a Pellucidar novel, this links Tarzan, Barsoom, and Pellucidar.
Gridley is also mentioned in the Amtor (Venus) series, linking those five books as well.
Tarzan is mentioned by the narrator as having participated in some historical event prior to the main story of the first story. Even then, the Tarzan books, and by association, everything Tarzan had appeared in, were part of the Amtor universe within the first twenty five pages of Pirates of Venus.
Also, the technology for the Moon mission from The Moon Men was Barsoomian in origin.
And Tarzan is a supporting character in The Eternal Lover, whose central character is the sister of the hero of The Mad King; thereby bringing those otherwise non-series novels into the fold.
Terry Brooks' Shannara series was always established as being set a fantasy world that formed After the End of modern civilisation. The Genesis of Shannara series is set during the collapse of civilisation, and establishes the past of the Four Lands as the Urban Fantasy setting of his The Word and the Void novels.
H. P. Lovecraft's story The Dream-Quest of Unknown Kadath ties most of his early standalone short stories into the Dreamlands Cycle, and also brings in Pickman's Model and the Randolph Carter stories. At the end, the Dreamlands Cycle is linked to the Cthulhu Mythos, though a few stories (such as the early "Dagon") may be outside the grand continuity. Several other authors have expanded this, notably August Derleth and Clark Ashton Smith.
Lovecraft also has a bit of a habit of tying his stories together by simply referencing passages from the Necronomicon, other forbidden books, or placing offhand comments during the expository monologues, about various Eldritch Abominations that have no bearing on the current story.
Cthulhu, Azathoth, and Yog Sothoth are mentioned this way all the time.
Even the weird demonesque race beneath the Earth from "The Rats in the Walls" seems to be referenced in "The Whisperer in Darkness" despite seeming to be completely unrelated.
This was essentially the point behind the "Mythos". Lovecraft would sprinkle references to it in otherwise totally unrelated works (and hoped other authors would do the same, and add to it) in order to give the impression of a deep and ancient mythology.
Beginning with It, Stephen King began tying many of his novels into The Dark Tower series, to the point that every single novel he wrote during the early 2000s was somehow related to the epic. The process included bringing back a character he Put on a Bus (literally) in 'Salem's Lot and retconning the Big Bad from The Stand into the Crimson King's Dragon. (Indeed, the Crimson King himself made his first appearance outside The Dark Tower series.)
From Desperation (1996) to From a Buick 8 and Everything's Eventual (2002), 100% of King's fiction output (six novels and two story collections) tied into The Dark Tower (at least retroactively). These were bookended by Wizard and Glass in 1997 and the conclusion of The Dark Tower series in 2003-04. Plus the aforementioned incorporation of everything back to Salem's Lot and The Stand, written before The Gunslinger.
And lest we forget, 'Salem's Lot takes place in the same city as Jerusalem's Lot, an earlier short story, confirmed to be in the Cthulhu Mythos. Therefore, The Dark Tower series is part of the Mythos by extension. Oh and as mentioned above Transformers, Doctor Who, Sherlock Holmes, and pretty much everything else on this page has crossed over with the Cthulhu Mythos.
It's also been established that if there's anyone in a King story with the initials R.F., they're probably a very particular person: Randall Flagg, the Big Bad of The Stand, The Eyes of the Dragon (as Flagg, no first name), and the Crimson King's Dragon. Except for (presumably) Rudy Foggia of The Jaunt, who is quite dead at the beginning of the story.
It also contains an appearance by Charles Pickman, from the H. P. Lovecraft story Pickman's Model — which ties it to all the Lovecraft stories mentioned below. King's next novel, The Tommyknockers, not only crossed over with It, but also tied in several of King's other novels, including Firestarter and The Talisman.
Also, the books Dolores Claiborne and Gerald's Game refer to each other as the female protagonists of the books have a psychic link, having times when they suddenly get the feeling that this other person, who they don't know, is somehow in danger.
Misery refers to The Shining at one point, when Annie mentions the ruin of the Overlook Hotel.
Tony Hillerman once had two series, one featuring Navajo cop Jim Chee and one featuring Navajo cop Joe Leaphorn. There is now only the Leaphorn & Chee Mysteries.
Though to be fair, from the beginning the Chee stories (which came second) would reference Leaphorn and characters and events from his stories—they just weren't featured in the same books for a while.
First The Poet and Blood Work got sucked into Michael Connelly's Harry Bosch universe, now Void Moon has been caught in the gravity well, too. Of course, unlike many of the other entries here, Harry Bosch's "world" is that of LAPD Homicide, and so referencing or including a few of Michael Connelly's stand alone novels doesn't really require much in the way of a Retcon.
Before he's done, F. Paul Wilson's Adversary Cycle bids fair to weave in practically every book and short story the man has ever written.
Mercedes Lackey's assorted Urban Fantasy stories seem to be set in different continuities, until mention is made of the west coast elfhames (from the Bedlam's Bard series) in the SERRAted Edge novels, and of Tannim, the mulleted protagonist of the SERRAted Edge novels appearing as a bit character in his teens in Jinx High, a Diana Tregarde investigation.
Since Jinx High was Tannim's first appearance, and the Bedlam's Bard events were namechecked in the first SERRAted Edge novel, this one was evidently intended from the start, or nearly so.
Kate Elliott has confirmed that her new Crossroads trilogy of fantasy novels is actually a fictional story within the context of her earlier Jaran series of SF novels.
Peter F. Hamilton retconned several of his earlier SF short stories to be set in the same universe as his immense, later Night's Dawn Trilogy and published them in a collection called A Second Chance at Eden. However, he has avoided this phenomenon elsewhere and has created no less than three distinct SF universes existed at similar points in history, making it impossible for them to coexist in the same continuity.
Alastair Reynolds did something similar with several of his early SF short stories, retrofitting them into his Revelation Space series of books and publishing the results as a collection called Galactic North.
Guy Gavriel Kay's The Fionavar Tapestry presents the world of Fionavar as so significant that echoes of it appear in the mythologies of every other world in The Multiverse. His subsequent stand-alone novels Tigana and The Lions of Al-Rassan, although each set in a different world, each has a moment showing that to be true. Ysabel is more overt, actually featuring several characters from the Fionavar Tapestry later on.
The Peter David novel Howling Mad mentions Mayor Penn, who is the returned King Arthur from Knight Life.
A particularly confusing example is The Well of Lost Plots, which ties the world of Thursday Next into a book (now a series) that Jasper Fforde wrote first, but which was published afterwards (The Big Over Easy, originally Nursery Crimes), and does so by establishing it as fictional within the Nextiverse, although, like all works of fiction, Thursday can enter it, and spends most of the book inside it, being ultimately responsible for its odd mix of genres. Everyone follow that?
To further confuse things, the Thursday Next stories are themselves fictional within the Nursery Crimes series.
Agatha Christie's Author Avatar Ariadne Oliver seems to tie several of her series together. She originally appeared in the Parker Pyne stories (as did Miss Lemon). Then she became established as a Hercule Poirot character, starting with Cards On The Table (which also featured Superintendent Battle, who'd previously appeared in the two novels starring Bundle Brent). Then she was the main character in the 1961 novel The Pale Horse, which also featured the vicar's wife from the Miss Marple novel The Moving Finger. And in Murder In Three Acts, Poirot meets Mr Satterthwaite, who previously appeared in The Mysterious Mr Quin collection of short stories. Tommy and Tuppence are also linked, since the same slightly unhinged old lady appears in The Pale Horse, the Miss Marple novel The Sleeping Murder, and the Tommy and Tuppence novel By the Pricking of my Thumbs, despite Partners in Crime having them refer to Poirot as a fictional character.
Tommy and Tuppence can also be linked to the others through a mysterious character who is only referred to as Mr. Robinson. This character appears with Poirot in Cat Among the Pigeons, Marple in At Bertram's Hotel, and Tommy and Tuppence in Postern of Fate. He also appears in Passenger to Frankfurt, which does not feature any of Christie's series detectives.
While Kim Newman has seeded connections between his books since the beginning, the short story "Cold Snap" seems to be a concentrated effort to tie them all together. A "Diogenes Club" story (and therefore featuring characters whose Alternate Universe selves appear in the Anno Dracula novels) it adds characters from his early work such as Jago, and even features the villain from his Doctor Who novella Time And Relative.
Under the pseudonym Jack Yeovil, Newman wrote a number of books based on Games Workshop properties. Krokodil Tears, one of the Dark Future books, had its Big Bad have a vision of an alternate version of himself as the Big Bad from his Vampire Genevieve series of Warhammer books.
Kim Newman isn't the only author to tie his personal Verse into the Whoniverse. Iris Wildthyme was a character in Paul Magrs' Magic Realism novels, before he revealed she was an extremely eccentric Time Lady.
Iris Wildthyme, in her appearances in novels and audios, occasionally interacts with an organisation called MIAOW, The Ministry for Incursions And Other Wonders (simultaneously a parody of Torchwood and Doctor Who's UNIT). This organisation has also turned up in his Brenda and Effie series of novels set around Whitby. Charaters from his Eighth Doctor Adventures novel Mad Dogs and Englishmen, his Big Finish Doctor Who audio drama The Boy That Time Forgot, and the Pheonix Court series that featured the original version of Iris have also appeared in this series. A character from one of Magrs' Tenth Doctor novels also reappeared in an Iris Wildthyme short story, along with a character from the Brenda and Effie series.
E. E. “Doc” Smith's Lensman series of novels was originally 4 books long (initially published in serial form in an SF magazine). In the late 1940s or early 1950s, he took an early work of his named Triplanetary and retrofit it in with the rest of the Lensman universe. He wrote an additional novel, First Lensman, to bridge the gap between the two storylines.
David Gemmell has stated that all his books take place in the same world, despite covering vastly different territory, such as a low-magic fairly standard fantasy world (Drenai Tales), a post-apocalyptic world (The Jerusalem Man) and our own world (an Arthurian duology and a duology set in ancient Greece).
The ninth Fighting Fantasy gamebook, Caverns of the Snow Witch, took the player on a tour of the major locations from two of the previous books (Warlock of Firetop Mountain and Forest of Doom), establishing that they all took place in the same land of Allansia. (It also name-checked a character from Forest of Doom.) The monster manual Out of the Pit then expanded this world: Allansia and The Old World, the setting for the Sorcery series of gamebooks, were two continents on the world of Titan.
Many, if not all, of the books written by Ted Dekker are in the same continuity, as one book references characters from seemingly unrelated books.
Leslie Charteris introduced Inspector Teal in the novel Daredevil featuring Storm Arden before Teal appeared in the Saint series.
Dale Brown has done this. Rebecca Furness and Daren Mace, characters originally in the non-Patrick McLanahan book Chains of Command, joined the main continuity in Battle Born and Warrior Class respectively. The eponymous space station of Silver Tower, thought a victim of Canon Discontinuity because of its long absence from his books, joins the main continuity in Strike Force. The Dragon of non-Patrick McLanahan book Storming Heaven, Gregory Townsend, is Dragon AscendantBig Bad of main continuity title The Tin Man.
Iain Banks, in his mainstream (non-SF) literature has said he doesn't do sequels/prequels; though he did include one subtle crossover in Complicity: Cameron's friend Al, an engineer he met on a paintballing weekend, is Alexander Lennox, recovered from his car-crash in The Bridge.
All of Christopher Moore's varied books appear to take place in the same verse, whether the setting is modern suburban California or Israel in Jesus's time. Various characters make appearances outside of their respective novels, like angels and vampires and fruit bats. Of course, whether this is actual canon welding or just his Verse depends on whether Moore had the broad strokes sketched out from the start or just made it up as he went (and tied it together afterwards)!
In a way, his series of books beginning with To Your Scattered Bodies Go could be considered the logical conclusion of this trope, as he intentionally designed a world in which he could bring in any character from any story written by anyone.
Madeleine L'Engle first connected her "Kairos" and "Chronos" series when Canon Tallis from Kairos novel The Arm of the Starfish appears in Cronos novel The Young Unicorns; several characters from each series would cross over later.
In The Art of Detection, Laurie R. King welds her wildly successful series about Sherlock Holmes' female apprentice to her lesser-known series about a modern San Francisco cop.
Simon R. Green's series The Nightside, Secret Histories, and Ghostfinders take place in the same world. And constantly reference each other. There are also very strong connections to his Deathstalker, Forest Kingdom, and Hawk and Fisher series. And all his other writings.
In the latest Nightside novel there's even a perspective-flipped recreation of a scene from a Hawk and Fisher novel, of the duo waiting at a tavern to meet Razor Eddie.
Narita Ryohgo wrote links estabilishing that his three series of light novels - Baccano!, Durarara!! and Vamp! - all take place in one Universe. For example, Shizuo from Durarara mentions getting into a fight with person strongly implied to be one of Baccano! characters.
It's not clear if this was intended from the start, but a minor character in the Starbuck series (set in the American Civil War) by Bernard Cornwall was revealed in the second book to be the son of Richard Sharpe, the hero of the Sharpe series, Cornwall's earlier and more famous series set in the Napoleonic Wars.
Neil Munro wrote two series of short stories for the Glasgow Evening News: Erchie, My Droll Friend about a Glaswegian waiter, and Para Handy, Master Mariner about a steamboat going up the West Coast of Scotland. When Erchie needs to take a ship to his daughter's wedding, naturally it's Para Handy's Vital Spark.
Andrzej Pilipiuk has connected his Jakub Wędrowycz stories with his more serious trilogy called Kuzynki (Cousins) - Jakub is mentioned by name in second volume and makes a cameo in third, combined with the illustration to leave no doubt that this is indeed him. This is odd, because in first book of the trilogy Jakub is clearly fictional as one of the characters reads his books and considers them the evidence that modern Polish literature is terrible.
E. F. Benson's Mapp And Lucia series only came together with the novel of that title, which brought the characters of Miss Mapp together with those of two previous Lucia novels. Although not regarded part of the series per se, another earlier novel Secret lives was also subsequently tied into the same continuity.
Anne McCaffrey wrote Pegasus In Flight and Pegasus In Space to officially merge the older To Ride Pegasus to The Rowan and the rest of the Tower and the Hive series.
Just about every book by Douglas Preston and Lincoln Child take place in the same fictional universe. They're probably best known for the Agent Pendergast series, but even their non-Pendergast books share characters that tie in with one another. For example, one of their earlier books, Thunderhead, introduced anthropologist Nora Kelly and featured William Smithback from their first Pendergast-related novels; Kelly was later made a recurring character in the Pendergast novels. Two Pendergast novels feature a wheelchair-bound profiler named Eli Glinn, he was introduced in an earlier novel entitled The Ice Limit and has later appeared in their new Gideon Crews series of novels. Mime, a hacker from the duo's second book Mount Dragon, has appeared in their later Pendergast novels.
Several of Piers Anthony's works. The last book of the Mode series featured a brief trip to Xanth. The 27th Xanth book included a visit to Phaze, a world from the Apprentice Adept series.
At the end of Christopher Anvil's "War With The Outs" series, humanity learns that beyond the Outs' territory, space is controlled by two new alien races, the Stath and the Ursoids. Both of these had previously made appearances in his "Colonization" series, suggesting that the "War With The Outs" stories take place earlier in the same universe.
Older Than Radio: French writer Honoré de Balzac wrote a few independant novels and short stories before making recuring characters. He next made the project of making a study of human society and called his work The Human Comedy (in reference to The Divine Comedy).
Robert E. Howard did this a lot with his historical, horror and fantasy stories. Just to name a few examples: Kull was explicitly tied with Conan the Barbarian in the essay "The Hyborian Age". Both was tied with the historical-fantasy character Bran Mak Morn through the Kull-Bran crossover "Kings of the Night". The ring of Thoth-Amon, from the Conan stories, and worshipers of Bran are featured in Howard's modern horror stories, while both Bran and Kull are mentioned in one of his Turlogh Dubh O'Brien stories set in 1200's. It wouldn't be unreasonable to consider all of Howard's speculative fiction to be part of the same verse, even if Howard never lived to point it out himself. And of course Howard and H. P. Lovecraft making references to each-others in their works was the foundation of the Cthulhu Mythos mention above.
Live Action TV
Russell T. Davies has suggested that Adam Mitchell's mum in Doctor Who, played by Judy Holt, may be the same person as Sister Mitchell in Childrens Ward, also played by Judy Holt, which would bring RTD's earlier programme into the Whoniverse. He was probably joking.
Seriously, the Doctor Who New Adventures novel Damaged Goods, which Davies wrote before being handed the series revival, has a scene near the end featuring a UNIT investigator who is implied to be the protagonist of his earlier series Dark Season.
Doctor Who has also crossed over with Quatermass, thanks to both being BBC creations and seminal British sci-fi. It started with a jokey reference to Quatermass' British Rocket Group in Remembrance of the Daleks, then the tie-in books made it explicit (with Quatermass turning up in one)...
The Tenth Doctor talked about Arthur Dent like he had met the guy.
The Doctor: Very Arthur Dent. Now, there was a nice man.
Shortly after the Torchwood base in Cardiff was blown up in the series' third season, a confused pterodactyl was picked up by the British Sanctuary. Probably just coincidence or a shout-out, but still: That beast was never seen or mentioned again on Torchwood...
Rumors abound to this day that Patrick McGoohan's Number 6 from The Prisoner is the same character as John Drake, his role in the earlier series Danger Man. McGoohan always denied it while other people involved in the show supported it, in what was probably a deliberate attempt to screw with the fans some more.
It's been suggested that McGoohan tended to deny it solely because he didn't hold the rights to his previous role, and thus, establishing a direct connection could be considered copyright infringement (and thus, potentially actionable). His co-writer on the series has always claimed that it was definitely Drake, though.
Although the Showtime revival of The Outer Limits was an anthology show, it usually ended its seasons with money-saving clip shows tying multiple prior episodes together into a single continuity.
It's been more-or-less established that all Nickelodeon sitcoms beginning with Drake & Josh all have some form of connection. Eventually enough crossovers happened for the Nick Verse to form. It is horrendously complicated by the fact that due to the way it came about, the actors and characters all exist alongside each other as real people. It includes Drake & Josh, Zoey101, iCarly, Victorious and also causes Big Time Rush and The Naked Brothers Band to enter the universe, via Miranda Cosgrove showing up As Herself on both those shows. This could somewhat more accurately (and awkwardly) be referred to as the Schneider-verse, for showrunner Dan Schneider.
Lisa Kudrow, who played a quirky waitress on Mad About You, played Phoebe on Friends. It was later revealed they were twin sisters and Ursula (the waitress) became a recurring character. It was also revealed that Paul once lived in the apartment now occupied by Kramer on Seinfeld.
In the second season of Friends, there was a brief crossover with Caroline In The City. Matthew Perry had a brief appearance on the Caroline episode as Chandler while Lea Thompson had an appearance Caroline in the Friends episode. Expanding beyond those two, earlier that same season Niles Crane and Daphne Moon appeared in one Caroline episode, and Frasier himself showed up in an episode of Wings. Based on this logic, potentially every NBC sitcom that ran during the 90's takes place in the same world (and also proceeds to directly tie into the Westphall example below)
Taking this to the extreme, due to various character cameos and crossovers, much of television history may take place in the mind of St. Elsewhere's Tommy Westphall.
The scariest part of that link? The X-Files crossed over with Cops, a reality show, meaning that all those shows take place in our universe... Which is, of course, the imagination of an autistic boy with a snowglobe.
An odd version of this exists in Dungeons & Dragons. Gods cross over from one campaign setting to another, spells exist under different names, and so on.
Spells also exist under the same names. Not just obvious names like Fireball, but ones like Bigby's Grasping Hand, which imply a specific creator.
The Planescape and Spelljammer meta-settings provided a mechanism for crossing over between published campaign worlds. Spelljammer showed that most of them existed in different solar systems of the Prime Material Plane, encased in crystal spheres, and one could travel between them in skyships called spelljammers. Planescape takes place mostly in the Outer Planes, but allows for portals to any Prime Material Plane world.
There was also the World Serpent Inn, which even links campaign settings which are explicitly not part of the Planescape/Spelljammer cosmology, such as Eberron.
Ravenloft is, itself, a product of Canon Welding, as its Patchwork Map incorporates several domains that were inspired, copied, and/or outright stolen from other AD&D campaign worlds. Literally stolen, in some cases.
The "legendary" settings of the various AD&D Historical Reference books were eventually revealed - in the appendix to Chronomancer - to be the past of Gothic Earth from Ravenloft: Masque of the Red Death...which in turn may the past of one of the magical d20 Modern settings - probably Shadow Chasers (the Red Death gets mentioned in the Menace Manual).
The Old World of Darkness was originally a set of unrelated tabletop RPGs which shared the same basic gameplay. Then White Wolf decided to link them all together, with rather strange results. While werewolves could reasonably fit into the same setting as vampires or mages, trying to jam vampires and mages into the same setting was a trick endeavour, given that both groups were said to have been secretly manipulating human history since the dawn of time. The new version is made with the possibility of such crossovers explicitly in mind, at the same time keeping each group at arm's length - the storyteller is not required to have them all exist if she or he doesn't want to, but the crossover rules ensure there'll be few to no snarls if they do. For example, the Supernal Realms of Mage: The Awakening and the Shadow World of Werewolf: The Forsaken have little to do with each other, but equally don't step on each other's cosmological toes.
The Shadow World is also explicitly part of the Mage cosmology. The only problem we currently have with the cosmology is the "Two Arcadias" hypothesis. In Mage, Arcadia is the Supernal Realm of Time and Fate, and separated from the human world by the Abyss, a massive rent in reality; while in Changeling: The Lost, Arcadia is a realm where humans are kidnapped off to and transformed into Changelings. Some believe that these are two different realms, while others believe they are the same realm. The books state that the answer is to be determined by the Game Master, but offers suggestions for both options. It's also possible that both are true: there is a "Fallen" Arcadia and a "Supernal" Arcadia which were originally one realm and are now separated by the Abyss, but the Watchtower of the Lunargent Thorn bridges the gap and allows them to intersect and interact.
Exalted was an inversion. The original concept for the game was for it to be set in the forgotten, mythical prehistory of the Old World of Darkness... but it was ultimately decided not to make this an absolute of the setting, and reduce the connections to common setting elements and parallels that hint at the possibility. The tagline "Before there was a world of darkness..." is The Artifact of the original concept.
Rifts. Want Robotech mecha to fight the Teenage Mutant Ninja Turtles alongside unicorn-riding cyborgs, only to have them all ambushed by eldritch abominations? Rifts. Palladium games specifically published conversion books for incorporating their other franchises into Rifts rules.
Which is mostly a case of converting some things to MDC. Other than that, every Palladium game uses the same basic rules. Another bit of Canon Welding comes in-universe. Hints have been dropped in the books that Rifts Earth is either a future version of Beyond the Supernatural, Heroes Unlimited, or a bizarre combination of the two.
The Warhammer and Warhammer 40,000 universes used to be linked, although the linking statements were made by mad characters. The whole saga/background is told through an Unreliable Narrator anyway. Games Workshop has stated that the link is now done away with, since it was mostly silly anyway. Warhammer world used to be a planet in the 40k universe, surrounded by warp storms that made it inaccessible for the rest of the galaxy. Nowdays they exist in separate universes, but there appears to be a small link between them in the form of the Warp (the Chaos Gods are the same in each universe, and some people in Warhammer world have gotten visions of Chaos in 40 universe. For example, in Liber Chaotica: Book of Khorne, it's all but outright stated the author is having visions of Abaddon's 13th Black Crusade. Also the Old Ones in Warhammer appear to be the same as the ones in 40k, and a fan theory suggests they escaped from 40k universe to Warhammer one after the War in Heaven). There is no real interaction between the two universes, however, unless you count some daemon characters popping up in both universes and a few magic items that have a suspicious resemblance to 40k technology.
It also used to be fairly heavily implied that Sigmar (the fantasy Empire's messiah figure and founder) was one of the missing Primarchs (genetically enhanced superhuman offspring of 40k's Emperor).
Warhammer 40 000 is now considered to be simply the Spiritual Successor where everything is an expy of the original Warhammer Fantasy.
The GURPS Infinite Worlds setting ties together every alternate universe they ever came up with and every licensed work ever adapted to GURPS from Uplift to Discworld to Hellboy.
The Green Hornet was the son of The Lone Ranger's nephew back when the two were on the radio. However due to legal issues between those who currently own the two franchises, the connection isn't used at all anymore.
When SNK made Art of Fighting 2, they decided to officially make the Art of Fighting series part of the same continuity as the Fatal Fury series. To explain why the Art of Fighting cast were not around during the events of the Fatal Fury games, they made the Art of Fighting series a prequel to the Fatal Fury series by setting it ten years before and putting a young Geese Howard as the True Final Boss in Art of Fighting 2 (back when he was still the police commissioner of South Town). When SNK later wanted to cross the Fatal Fury cast with the Art of Fighting cast in The King of Fighters games, they had to place the third series in a separate continuity.
And from there, it starts getting really weird, with Ralf and Clark from Ikari Warriors and Athena from Psycho Soldier (who is the descendant/ambiguously-the-reincarnation of the Athena from Athena) appearing in The King of Fighters — despite Psycho Soldier involving an invasion of monsters from beneath the Earth that you'd kind of think would get mentioned at some point in KOF canon if it happened — and then both Ralf and Clark and KOF original Leona appearing in Metal Slug 6 and Metal Slug 7. At this point, the only sane response to the SNK canon is to throw up your hands and shout "I don't know!"
Ivalice Alliance series was the first attempt to make a Final Fantasy universe that included more than one game and in fact includes games without the Final Fantasy name. Final Fantasy Tactics, the Final Fantasy XII titles, Vagrant Story, Crystal Defenders, and the not yet releasedFortress are among the comfirmed games to take place in the Ivalice universe.
When the Czech developers of Operation Flashpoint: Cold War Crisis (their first game) Bohemia Interactive Studios split with the publisher, they were able to keep the "assets" (i.e. the game engine) but not the name, so they ended up creating a Spiritual Successor called ARMA. However, BIS has since come out and said that both series are in a Shared Universe, as will be BIS' spin-off Take On Helicopters.
The latest patch for Operation Flashpoint even renames it to ARMA: Cold War Assault. Talk about paying attention to details.
This started off as a Shout-Out in the original Saints Row, with a few references to Ultor here and there. Then Saints Row 2 ran with it, and made Ultor a major faction that is clearly the same one as in Red Faction.
The sheer amount of shout-outs to NiGHTS in the Sonic the Hedgehog series has led some people (including those of semi-official status) to claim they are in the same universe.
The Valve series Half-Life and Portal share a universe, as hinted at by references to Black Mesa in the original Portal, and some vague comments by GLaDOS in reference to the Combine, and verified by the appearance of an Aperture research ship toward the end of Episode 2, and an explanation of Aperture and Black Mesa's backstory during the old Aperture levels of Portal 2.
Commander Keen and Wolfenstein 3D are linked together by way of Commander Keen's Billy Blaze being the grandson of Wolfenstein's B.J. Blazkowicz. A common fan theory is that the protagonist of Doom is also part of the family. The RPG versions of their respective games go with this - the "Harbinger of Doom" from Wolfenstein RPG is basically Doom's Cyberdemon without prosthetics, and the hero of Doom RPG is explicitly given the last name "Blazkowicz".
Ever since 2004 Ninja Gaiden has been established to be in canon with Dead or Alive series with Ryu being the canonical winner of the second tournament and Dead or Alive character Ayane acting as a support character throughout many Ninja Gaiden installments.
Up until Fire Emblem Awakening, it was assumed that barring remakes and direct sequels/prequels, all Fire Emblem games were set in different universes and did not impact each other. However, Spot Pass and DLC content for Awakening used characters from previous Fire Emblem games -Including ones that were not set in the Archanea universe- and so to get around this, introduced a "gateway" that allowed characters to travel between the different universes. This would imply that all of the worlds are connected, physically or otherwise, as part of some greater universe rather than separate, alternate worlds.
It was initially believed that BioShock Infinite was an In Name Only sequel to BioShock and BioShock 2. It's now been revealed that the many alternate universes in which Infinite takes place are part of a multiverse "neighbourhood" of realities that all begin with a man approaching a lighthouse- one of which is that of the first two BioShock games. For good measure, you actually visit Rapture at the end.
Discussed in Project × Zone where (on top of canonizing about a dozen other crossover games) the characters from the many different universes have some fun trying to link them together. For instance, the characters of Gods Eater Burst find it plausible that their world would continue on a downward spiral, necessitating the construction of Basel in their far future. However, it's also acknowledged that certain universes are mutually exclusive with each other (Sakura Wars is technologically and geopolitically incompatible with modern earth) or completely isolated (Tales of Vesperia is shown to be in its own disconnected bubble), so nothing is set in stone.
The MS Paint Adventures series Problem Sleuth was tied into the earlier Jailbreak series when Zombie Ace Dick and his whale crashed into the jail where Jailbreak was set. Indeed, a dead whale was part of an early Jailbreak puzzle, and ZAD and the Completely Sane Man were revealed to be the skeletons in one of the cells.
In a borderline example, in a semi-canonical donation extra the Problem Sleuth characters did battle with the Midnight Crew. That gang would later become extremely plot important in Homestuck. Even though both Problem Sleuth and the Midnight Crew exist as fictional works in the Homestuck universe - in fact, instead of Homestuck, in-universe the adventure following Problem Sleuth was based on The Midnight Crew. Yeah.
In Starslip Crisis, the character of Vore is all but explicitly stated to be in fact Vaporware from the author's previous comic, Checkerboard Nightmare. However, this can be considered only to be a partial example, since said strip's events are never mentioned in Starslip and Vore himself seems to have lost his memory up to that point, causing a bit of a personality change (yes, Vaporware also expressed desires to exterminate mankind, but Vore's a lot more proactive about it), so for all intents and purposes Vore can be considered a separate character. Eventually he did regain his old memories and personality, and started calling himself Vaporware again...right before he was killed off for real.
On the other hand, records of the past (or Real Life, 21st Century Earth) seem to be extremely sketchy, as evidenced by the Show Within a Show "Concrete Universe," where covered wagons exist at the same time as cloning.
Crossoverkill fueled the fire, not only adding more webcomics to The Multiverse, but also with revelation that Doppleganger Gang members are all alternate reality counterparts of one another and there are members from viarous webcomics among them, including several fantasy webcomics.
Heroes Unite did it with a horrifying amount of Super Herowebcomics, hosted on Drunk Duck. First it estabilished that Energize, Bombshell, and an alternate counterpart of Acrobat share an universe, and then a bunch of other superheroes joined in. Some writers even took an advantage of it to make their webcomics more popular. The creators of Energize and Dasien did a short (currently on hiauts) crossover between their characters, while the former used a new Shared Universe to bring back his other webcomics - Fearless, SHELL teamed up with The Blonde Marvel and Bombshell and gets his ass kicked by one of Hero Force members before joining HU, and Vora, Princess Of The Skies, appeared a few times in HU before getting her own adventures. And it's all one reality in the webcomicsmultiverse.
T Campbell has done this with various webcomics he's written or co-authored, both played straight and using alternate versions of characters.
Penny and Aggie, Cool Cat Studio and Sketchies are set in the same universe. However, the SF and supernatural elements in Cool Cat Studio are absent from the other two comics. Campbell once explained this on the P&A forum by stating that such elements exist on the periphery of the comics' shared universe, so not all its inhabitants experience, nor are even aware of, such things. This is in contrast to the Fans! universe (and its alternate versions of P&A's characters), where, particularly after the Revival, paranormal occurrences are so frequent and prominent that the entire world is aware of them.
Also alternate versions of characters from Penny and Aggie and Fans! appear in each other's universes every so often.
Barry T. Smith's Ink Tank appeared to be in an entirely new universe from the previous strips...until a story arc which ended with the Author Avatar having a nervous breakdown was resolved by Dante from Angst Technology turning up and treating him to a coffee.
Artist Ursula Vernon's Digger, an anthro adventure about a mildly cynical wombat and a statue of the god Ganesh, has this if you start reading her other work. An awful lot of everything she's done seems to have characters in common with the Gearworld, her vaguely-steampunk clockwork-labyrinth art-and-fiction setting. It's only vaguely hinted at in Digger itself.
Sugar Bits might have done it when one of the villains summoned Red and Big Bad Wolf from Ever After to fight protagonists. However, given the nature of the Sugar Bits world and Bleedman's own words, those two comics may or may not share an universe and this will remain unresolved until Endling, creator of Ever After, will confirm it.
Shaenon Garrity's Narbonic and Skin Horse were officially confirmed to take place in the same continuity with the introduction of Artie Narbon to Tip Wilkin. Garrity had previously revealed in Narbonic Director's Cut that the main Narbonic characters, Dave, Helen, and Mell, came from three different comics she had drawn in high school and college. Mell also gets her own Spinoff Babies comic, Li'l Mell, and a character introduced in that comic has now shown up in Skin Horse. Garrity's lesser-known Smithson may fit into the same continuity as well; minor character Queensbury Joe appears to be the older version of Homeschool Joe from Li'l Mell.
Mel has also appeared twice in Everyday Heroes - once in a brief flashback where Mr. Mighty thwarts one of Helen's capers and again where she appears as Dr. Unpleasant's lawyer.
Glorianna and Sparky of Lady Spectra And Sparky shared an adventure when Glorianna was briefly transported to the 21st century (while Lady Spectra ended up in Glorianna's era).
The (In)Famous David Gonterman, Ed Wood of the World Wide Web and the Internet's Most Dangerous Cartoonist; reuses characters, concepts and names so often that it more or less seems that every single thing he's ever written or drawn are all set in the same vague universe/multiverse. This may be partially intentional, but, well...
There was also an episode of Hercules in which Jafar and Hades cut a deal to eliminate each other's enemies, resulting in an AladdinCrossover.
It should be noted that the crossover fit nowhere into the Aladdin timeline, so these were possibly AU versions (not to forget that the world in Aladdin is shown to be round, while in Hercules it's more or less flat).
The Kingdom Hearts series of video games has most major Disney worlds, or rather, explicitly AU versions of them, existing in a larger multiverse.
In the South Park episode "Super Best Friends", there is a brief scene of the cast of That's My Bush! in the White House, indicating that the two series (both created by Matt Stone and Trey Parker) take place in the same universe (which is particularly silly, since That's My Bush! is a live-action show).
There were reportedly plans to weld the canons of Exo Squad and Robotech back when the former was produced. Considering that their primary motivation seemed to be that both shows were Merchandise-Driven, your mileage may vary on whether it's a good or bad thing that this never happened. It went beyond just planning; there were figures of Robotech mecha sold in Exo Squad-branded packaging.
American Dad!s Stan and Bullock try to stop Family Guy's Stewie plan to take over the world, with Stewie mistaking Stan for Joe. Of course, this takes place in a virtual reality simulation, so it's debatable weather it's canon or not. There have been a few other smaller cameos.
There is a deliberate lack of crossovers between Family Guy and American Dad but they're considered to be part of the same universe. However at the end of the American Dad episode "Hurricane!", the houses of Cleveland and Peter end up on the both sides of Stan's house resulting in them going into a stand-off that ends with Francine accidentally being shot by Stan. It's never addressed in later episodes of any of the shows.
ThunderCats (2011), as of the episode "Legacy" features new versions of characters from SilverHawks and TigerSharks, all of which were action shows produced by Rankin/Bass in the '80s, sharing the same art style, writers, and voice actors. While the shows were incredibly similar and could easily be mistaken for being part of the same universe, it's only official now after twenty plus years.
Rankin/Bass Productions is also famous for its adaptations of Christmas stories, and eventually welded many of them together in a movie called Rudolph and Frosty's Christmas in July. Naturally there were a lot of elements that did not quite fit together—Santa Claus, for example, had a subtly different appearance and personality in each previous special—so decisions and adjustments were made. Likewise some scenes from Rudolph and Frosty's lives were shown that differed from their own specials, but kept the basic facts the same.
The Critic Jay Sherman once showed up to judge the The Simpsons Springfield Film Festival, though Matt Groening was so against the idea that he took his name off the opening credits for that episode.
The Transformers have already been listed, but deserve special mention here; in the '80s, it was broadly hinted that the original cartoon shared a universe with the G.I. Joe cartoon, and not-quite-as-broadly that the same was true for Jem and Inhumanoids. This has been taken much further in the 21st century, where not only were the previous hints confirmed, but now every Hasbro cartoon from the time period has been stated to be part of the same universe, and pretty much every other Hasbro property exists within the Transformers multiverse.
The show's tie-in comic (which was written by the show's creators) blatantly contradicted this, showing versions of Scarlet Witch and Quicksilver that were nothing at all like their WATXM counterparts.