When two or more works, be they books, movies or comics or two completely different media altogether, exist in Shared Universe or Expanded Universe, you are expecting them to be bound to the rules that come with this fact - when something big, involving all known worlds happens, you are expecting this event to be referred to in several titles. For example, when somebody dies in one work, he wouldn't appear in another, and internal rules like magic apply no matter which work you are reading, watching, playing, or listening to.
Well, this isn't always the case. Maybe the writer decided that Canon Welding was a bad idea in retrospect. Or he/she didn't want to feel limited by what happened in another title. As well as the possibility of the writer wanting to take the works in a different direction, or that the writer was afraid being too continuity-heavy would discourage assorted potential readers. Whatever the reason may have been, the writer doesn't really act like the titles currently are in one continuity.
In order to avoid several kinds of problems that could come from not sticking to one continuity, some writers may decide to announce that two or more works are still in continuity and are not, From a Certain Point of View.
There are a few varieties of this trope:
- One-Sided Continuity: Work A is in continuity with Work B, but Work B is not in continuity with Work A. So works from one title are still canonical and its characters may show up in the other, but not the opposite. It might be explained that characters from Work A exist in their own, separated world, and Work B is an Alternate Universe, where their counterparts exist, but their adventures may go different ways and they may meet characters that may not even exist in the title they originate from.
- Alternate Timeline Continuity: Work A was always in Alternate Continuity for Work B, but the timelines were identical until a certain point, at which point they diverged. Sub-Variant of One-Sided Continuity, chances are, this will be used either when two or more titles start going in two or more different ways, or in case at least one of them is set in a different time from others. Going from this, it would be that one or more of the works happening later on the timeline are a possible future and things don't have to turn out like they did in it.
- Bait-And-Switch Continuity: Work A and Work B don't take place in one continuity, unless said otherwise. In other words, two titles have their own, separate settings, but whenever the writer wants to, they can meet and the crossover in question will act as if and were always set in one continuity. Canon Welding is however averted, because once the Crossover is over, assorted characters act like it never happened. Needless to say, this is can be extremely jarring for more continuity interested fans, who wouldn't accept that and will try to explain how two titles can exist in one world, even if several works aside from crossovers contradicts that.
- Divorced Continuity: Work A and Work B existed in the same world, but something split it in two. This variant is similar to Alternate Timeline, but there was an event in-universe that had resulted in separating two worlds - all the history from one work has been removed from the history of another and vice versa, the characters never existed in one world and nobody remembers them as their stories exist and always existed separately. Alternatively, in order to avoid creating Continuity Snarl, Expies are created to fill the roles of deleted characters.
Examples of One-Sided Continuity:
- The My Little Pony G4 cartoons, Friendship Is Magic and Equestria Girls. They share the same characters and the same writing staff, and these writers have insisted that the events of Equestria Girls actually are canon to Friendship Is Magic. But the character arc in EQG is completely dependent on a big event from FIM season three, while FIM's only reference to EQG have been a few blink-and-you'll-miss-it cameos. So even though EQG is technically canon, it's totally ignorable for the fans who are so inclined.
- This also extends to the comics. The events of the cartoon have happened, mostly, but the comics have yet to be referenced in the cartoon, with a few episodes strongly implying that the comic stories haven't happened, and members of the staff have said the comics won't effect or be referenced in the show.
- Disney comics in general Broad Strokes versions of most of the Walt Disney Classics exist within the comic universenote , but don't expect to see any anthropomorphic characters in sequels and merchandise for these.
- Star Wars and its Expanded Universe had this relationship under George Lucas - movies were in canon with all books, comics and games, but Lucas refused to acknowledge the opposite.
- Ditto for Star Trek. The TV series and movies are considered canon by the novels and comics, but not vice versa. Unlike the Star Wars Expanded Universe, however, Trek's various comic series and individual novels were not considered in canon with one another, until Star Wars popularized the idea in the 2000's and Trek followed suit with its novels. note
- Sony's Universe of Marvel Characters and the Marvel Cinematic Universe might be a case of this. Sony owns the rights to Spider-Man characters, but made the MCU films Spider-Man: Homecoming and Spider-Man: Far From Home in conjunction with Marvel Studios. While Marvel has specifically stated that Venom (2018), starring a popular Spider-Man opponent, is not in continuity with their shared universe, Sony has made comments implying that they are. (Or at least, in continuity with Homecoming, which is too connected to other MCU films to easily write them off.)
- In a similiar manner, for the Gundam franchise only the animated works are considered part of the various continuities, and works such as the novels written by Gundam's creator Yoshiki Tomino are not acknowledged as part of the continuities of the shows. This especially includes Mobile Suit Crossbone Gundam, despite it also being written by Tomino.
- Acrobat had been a part of Heroes Unite until the former's creator announced that his characters appearing in latter and related titles are not the originals, but their alternate counterparts.
- Sugar Bits and Everafter had a crossover within the pages of the former, but Bleedman has let the latter's creator decide if he wants them to share The 'Verse. It's hard to ignore all pages with Red and Big Bad Wolf in Sugar Bits, so it's safe to say that Everafter exists in the Sugar Bits world, but the opposite may be not true.
- Mindmistress and Zebra Girl were established as existing in one Universe, but Word of God from the creator of Mindmistress is that the Zebra Girl from her world may not be the original one, but an alternate version.
- In similar manner, two webcomics that established a large Multiverse do it: The Crossoverlord and Crossoverkill both portray The Order of the Stick as existing in that multiverse, but Word of God says that unless Rich Burlew will agree with that, it's only "one of many worlds working on similar rules and laws physics as The Order of the Stick". In general, Canon Welding done by the two series may result in this if creators of comics dragged into The Multiverse don't want them to.
Examples of Alternate Timeline Continuity:
- The second-edition Trinity Continuum settings work this way, because they're four games set in separate time periods of the same universe, but the decision was made to not have a set future for any individual game. As a result, if you're playing in a chronologically-later game, then the earlier games are considered canon, but if you're playing an earlier game, later games are only "possible futures" and nothing is canon past the start date.
- Word of God states that The Witcher games are just one possible future for The Witcher book series. This is in case Sapkowski would want to write a sequel to the series, and so he wouldn't have to reference the games.
- Grim Tales from Down Below is said by Bleedman to be only a possible Bad Future for his other webcomic, Power Puff Girls Doujinshi.
- Happened with two popular Warhammer40000 fan fictions - in The Age of Dusk the Necrons unleash a devastating attack on the Tau, almost destroying their empire and entire race, in order to prevent the possible future known as Rise Of Tau from happening.
Examples of Bait-And-Switch Continuity:
- Image Comics operates on this rule - their comics exist in one world only when crossovers takes place and events from one series may be referenced in another only if the creators wish it. It's especially visible in Invincible #60 — comics that featured appearances of probably every single Image Comics hero aside from Bomb Queen and you won't find a single reference to those events (including the destruction of several cities) anywhere but in Invincible and The Astounding Wolf-Man.
- Akira Toriyama's Dr. Slump had a tendency towards Negative Continuity where Arale could split the world in half and have everything back to normal by the next chapter. His following series Dragon Ball actually featured the cast of Dr. Slump in one of the storylines, despite much of Slump's logic or continuity (or lack thereof) making little to no sense in the context of Dragon Ball, the whole encounter playing off almost as a Big-Lipped Alligator Moment to anyone not familiar with his original series.
- Most inter-company comic book crossovers worked like this until 1996's DC Versus Marvel. Prior to that miniseries, when Superman met Spider-Man, the Teen Titans met the X-Men, or Gen13 met Spider-Man, the assumption for the duration of those stories was that the two characters had always been part of the same universe and had simply never met before. Since DC Versus Marvel, such crossovers have been much more likely to involve parallel universes.
- The Simpsons has had crossovers with The Critic (downplayed in that the only elements from that show to appear are Jay Sherman himself — whose character design was tweaked to fit in with The Simpsons — and Coming Attractions), Family Guy (which poked fun at their different art styles a few times), and Futurama (which ignored the obvious skin color differences despite the two shows having the same art style), but since the show mostly runs on Negative Continuity, they leave little impact on the show itself (with the notable exception of a deactivated Bender appearing briefly in "Cue Detective"). Notably, characters from both shows made a few cameo appearances in each other before their crossover episode, but they're just in the service of throwaway gags, and the crossover episode itself depicts them as never having met before.
- Each season of Super Sentai and Kamen Rider is largely standalone, but whenever there's a crossover all bets are off. What makes things confusing is that sometimes the two series/seasons are explicitly crossing between Alternate Universes to meet each other (Kamen Rider Decade is the most obvious example), but sometimes they act as if they share a world even when their settings are completely incompatible (for example, Kamen Rider Ex-Aid, set in modern Japan, saw an out-of-nowhere cameo by Lucky of Uchu Sentai Kyuranger when Kyuranger is a far-future Space Opera where Earth has been under oppression for hundreds of years).
Examples of Divorced Continuity:
- Image Comics separated their original universe into a bunch of lesser ones in the Shattered Image event. Since then, each sub-publisher has its own universe that coexists with others in Bait-And-Switch continuity, with the exception of the WildStorm universe, which was bought by DC Comics, and Rob Liefeld's universe, for the period of time that he left Image.
- Dallas and its Spin Off Knots Landing had a shared continuity until Bobby's death on Dallas (Bobby's brother Gary was the main character on Knots Landing). Gary gets the news and it affects him; later in the season Gary's son is born and he names him Bobby. Come the beginning of the next season of Dallas we learn that Bobby's death (and the entire previous season) was All Just a Dream; but Knots Landing kept the plot point, and the two shows never interacted again.