[[quoteright:320:[[Webcomic/NerfNow http://static.tvtropes.org/pmwiki/pub/images/TrilogyCreep_802.jpg]]]]

->''"The fifth book in the [[LampshadeHanging increasingly inaccurately-named]] [[Literature/TheHitchhikersGuideToTheGalaxy Hitchhiker's Trilogy.]]"''
-->--''Literature/MostlyHarmless''

The strange tendency of trilogies to expand and see more and more works added to TheVerse. Mainly found in books, but may also occur with movies.

SF author Creator/OrsonScottCard has suggested that this is the result of ExecutiveMeddling; rather than allow an author to write the books he wants to, publishers pressure him into producing sequel after sequel in order to take advantage of the preexisting fanbase and milk a CashCowFranchise bone-dry. (Note he also made the distinction between genuinely expanding a story versus ''splitting'' a story up due to excessive length, such as was the case with his ''Xenocide'' and ''Children of the Mind'' - not to mention [[Creator/QuentinTarantino Tarantino's]] ''Film/KillBill'').

TwoPartTrilogy is somewhat related, typically the result of a one-part story expanding into a trilogy.

See also FranchiseZombie. CapcomSequelStagnation is a related trope for VideoGames.
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!!Examples:

[[foldercontrol]]

[[folder:Audio Plays]]
* In the AudioPlay/BigFinishDoctorWho "Companion Chronicles" series, "[[Recap/BigFinishDoctorWhoCCS5E2EchoesOfGrey Echoes of Grey]]" and "[[Recap/BigFinishDoctorWhoCCS6E3TheMemoryCheats The Memory Cheats]]" were supposed to be the first two parts of a trilogy. The CD notes for the third instalment, "[[Recap/BigFinishDoctorWhoCCS7E2TheUncertaintyPrinciple The Uncertainty Principle]]," make it clear that it is not the planned final part, and the 'trilogy' is now expected to be four episodes long.
[[/folder]]

[[folder:Fan Works]]
* ''WebAnimation/TurnaboutStorm'' is an interesting case. Originally a 4-part series, the episode count increased to five when ''Part 3'' was split into two episodes via SimultaneousArcs, but with the final part still being called ''Part 4'' -- At least until ''Part 5'', the real conclusion, was confirmed via {{Cliffhanger}}.
* In the case of Fanfic/TheCalvinverse, this happened to two different trilogies at the same time - Creator/{{Swing123}} and Creator/{{garfieldodie}} both made their own separate ''ComicStrip/CalvinAndHobbes'' trilogies, and ultimately ''Fanfic/RetroChill'' served as a {{crossover}} between the two (cementing the series as a universe, it could be said).
[[/folder]]

[[folder:Film]]
* The Film/AmericanPie trilogy, direct-to-video spinoffs aside, later got a fourth movie: ''American Reunion''. A fifth movie has been stated to be likely.
* One of the taglines for ''Film/ScaryMovie 4'' was "The Fourth and Final Chapter of the Trilogy". There's also the announced fifth movie. In France, ''ScaryMovie 3'' had the tagline [[CaptainObvious "Best trilogies are in three parts."]]
* ''Film/{{Alien 3}}'' is the definitive ending for the story of Ellen Ripley, who died by simultaneously falling into molten lead and giving birth to a Xenomorph queen. However, ''Film/AlienResurrection'' brought the character back as a clone who finally made it to Earth. Up until the release of the ''Alien Legacy'' box set in 2000, the first three movies were still packaged in one case as the "Alien Trilogy", with the fourth film packed in separately. It's also been released as a "Quadrilogy" and an "Anthology" (to say nothing of the spinoff ''Film/AlienVsPredator'' films or the 2012 pseudo-prequel ''Film/{{Prometheus}}'').
* The ''Franchise/IndianaJones'' trilogy was expanded with [[Film/IndianaJonesAndTheKingdomOfTheCrystalSkull a 2008 sequel]], with a fifth rumored about the adventures of Shia [=LeBeouf=]. Justified in that, like ''Franchise/StarWars'', was going to be a longer saga originally, with five or six movies. Things just kept getting in the way of development. In the meantime, much material was added to the ExpandedUniverse.
* ''Film/LiveFreeOrDieHard'' and ''A Good Day to Die Hard''. Something of a stretch, since the ''Die Hard'' series doesn't really have an overarching storyline anyway.
* ''SpyKids''. However, it's not really a continuation of the first three, since it's centered around a new family (the family of the previously-unmentioned aunt of the original spy kids, to be exact), though the original spy kids, now grown-up, do appear.
* The ViewAskewniverse, which started out as the Jersey Trilogy and became the Askewniverse Chronicles. There are six films released, with ''Clerks 3'' on the way.
* ''PiratesOfTheCaribbean''. However, the fourth film is an entirely new adventure featuring [[BreakoutCharacter Jack Sparrow]], rather than a continuation of the previous films' arc. Rumors are that 5 and 6 will be filmed back to back.
* ''{{Shrek}} Forever After'', the fourth ''Shrek'' film. They promised this'll be their last attempt to squeeze more dollars out of the green, Scottish CashCowFranchise. [[ExactWords Then they made the]] ''WesternAnimation/PussInBoots'' spin-off (which had [[DevelopmentHell already been in development for years]] when Shrek 4 went into production). Originally, five Shrek films were planned, but the franchise's decline in popularity following the negative reception of ''Shrek The Third'' seems to be the cause of Dreamworks shortening the saga to only four installments.
* ''Film/ParanormalActivity'' came out with a fourth film in an obvious attempt to [[CashCowFranchise get more money from the fans]]. Not surprisingly, both critics and fans agree that this is the worst one in the series.
* The original ''Franchise/StarWars'' trilogy covers a timespan of four years between the beginning of the first film and the end of ''ReturnOfTheJedi''. In the intervening years, a prequel trilogy, multiple television series, games, books and comics have come out. The timeline of events in the series now spans from over 5,000,000,000 BBY (Before the Battle of Yavin) to 138 ABY (After the Battle of Yavin). Not only that, but there are multiple "eras" (the ''KnightsOfTheOldRepublic'' franchise, the ExpandedUniverse sidestories, prequels and one-shots set during the OT, and the "New Republic", "New Jedi Order" and "Legacy" arcs) that have galactic power flip-flopping between the Rebel Alliance and various incarnations of the Empire. And then {{Disney}} brought the rights to the franchise and decided to do a ''whole new trilogy'', also declaring all ExpandedUniverse material non-canon.
* ''{{Saw}}'' was originally intended to be a stand-alone film. Because of the astounding success of that one, they decided to end on the third movie. Obviously, they came back after the third one and just decided to flesh out a story and keep writing until they came up with the perfect ending. They came up with an additional 5 scripts. The trope was slightly subverted when they had to cut down to 4 scripts because of the success of the Paranormal Activity franchise, which would later cause the death of the Saw franchise.
* The ''Film/{{Spider-Man}}'' films are a strange case. For a while, there was talk of making six movies, and 4 was even in development, but then of course thanks to CreativeDifferences, it stopped with ''Spider-Man 3'', which just happened to wrap up the current plot threads. The franchise was then [[ContinuityReboot rebooted]] with ''Film/TheAmazingSpiderMan''.
** Set to occur in the rebooted franchise, as well--Sony set out release dates for sequels to Amazing Spider-Man up to 4 ''plus'' two spin-off movies, before the second film was even released.
* George Romero's ''NightOfTheLivingDead'' film series stood as a trilogy for 20 years and became a hallmark of the zombie film genre before receiving a fourth installment in ''LandOfTheDead'', which got some great reviews but was viewed by some fans as a disappointment. Two more installments, ''DiaryOfTheDead'' and ''SurvivalOfTheDead'', came out in rapid succession, to very little cultural impact.
* ''Film/{{Scream 4}}'', though given that this is the ''Franchise/{{Scream}}'' series, it took a couple digs at this.
* An interesting case is the ''Franchise/StarTrek'' franchise. With six films based on the [[Series/StarTrekTheOriginalSeries original series]], four on ''[[Series/StarTrekTheNextGeneration The Next Generation]]'', a [[Film/StarTrek reboot of the original]] ''and'' a [[Film/StarTrekIntoDarkness sequel]] of that reboot, ''Star Trek'' ceased being a trilogy a ''long'' time ago. That said, ''[[Film/StarTrekIITheWrathOfKhan The Wrath of Khan]]'', ''[[Film/StarTrekIIITheSearchForSpock The Search for Spock]]'' and ''[[Film/StarTrekIVTheVoyageHome The Voyage Home]]'' are together now retroactively marketed as "The ''Star Trek'' Trilogy" and have been released together in a boxed set this way. They are also the only three films in the entire twelve-film series to follow one roughly self-contained story (though plot points do receive mention in the later films).
* The credits for ''Film/ViolentShit III: Infantry of Doom'' read "End of Trilogy". Cue another sequel, ''Karl the Butcher vs. Axe'', eleven years later.
* ''Deadly Dares: Truth or Dare Part IV'', made thirteen years after ''Screaming for Sanity: Truth or Dare 3''.
* ''Rambo'' (2008), which is more informally known as ''[[FanNickname Rambo 4]]'', arrived in theaters a full two decades after the final installment in the original ''First Blood'' trilogy that introduced the character to movie audiences. The fourth film isn't really that gratuitous, however, since it allows John Rambo some closure by having him, at the film's very end, [[spoiler: [[CrowningMomentOfHeartwarming finally return to his father's ranch in Arizona]], which he has been away from for close to 40 years]].
* [[Film/TheBourneSeries The Bourne Trilogy]] with its fourth installment, ''The Bourne Legacy'', where Jason Bourne is only named.
[[/folder]]

[[folder:Literature]]
* Frank L. Baum tried to end the ''[[Literature/LandOfOz Oz]]'' series at the sixth book, stating that there would be no way to ever contact the Land of Oz anymore. That didn't happen, of course, and he was forced to write several more books before handing off to Ruth Plumly Thompson.
* Creator/OrsonScottCard, who discussed this trope above, is no stranger to this.
** ''Literature/TheTalesOfAlvinMaker'': The fourth book of going-on-seven opens with a chapter-length rant from the author that's titled "I Thought I Was Done" and justifies its existence by meandering into and out of setting background.
** ''Literature/EndersGame'' began as a novella. When he tried to expand it into the novel ''Literature/SpeakerForTheDead'', he realized it would have a very slow, boring beginning--but if he stretched that beginning out even further, and turned it into a novel of its own, he'd be able to add details and character development and make it more interesting. Then his publisher accidentally wrote out a contract for "the Ender trilogy," and he had to [[DolledUpInstallment rewrite a planned standalone]] called ''Philotes'' into the third book. Then he realized just how long ''Philotes'' was, and split it into two books, ''Literature/{{Xenocide}}'' and ''Literature/ChildrenOfTheMind''. [[spoiler:Killing off the main character]] nearly ended the series, but then he realized the thousand-year TimeSkip between books 1 and 2 allowed for plenty of {{Interquel}}s. Now the series is up to double digits.
* GregoryMaguire's series [[Literature/{{Wicked}} The Wicked Years]] was three books, Wicked, Son of a Witch, and A Lion Among Men. It is now has a fourth called Out of Oz.
* ''Literature/TheHitchhikersGuideToTheGalaxy''
** Creator/DouglasAdams' "trilogy" eventually ended with the fifth book, ''Literature/MostlyHarmless''. He [[LampshadeHanging Lampshaded]] it by calling it a "trilogy in four parts" after the fourth book came out and an "increasingly inaccurately-named trilogy" after the fifth. The first three books go together quite well, and the last two seem a bit awkwardly tacked on. The fifth book seemed to be a case of TorchTheFranchiseAndRun, but he later revealed that [[CreatorBreakdown the ending was more due to his mood]] than anything and he regretted it. He would have un-torched the 'verse in a sixth book, [[AuthorExistenceFailure but alas, unfortunately]]...
** Eoin Colfer (writer of the Literature/ArtemisFowl books) has written ''Literature/AndAnotherThing'', making this a trilogy of six. The book emphasizes the idea of "no endings" throughout, and while it could be a conclusion to the series, it points furtively in the direction of an ongoing story. The book's cover states that it is "Part Six of Three".
** ''Literature/TheSalmonOfDoubt'', which contains excerpts from an additional ''Dirk Gently'' novel that Adams was developing at the time of his death, includes notes that he was beginning to feel ''Mostly Harmless'' was not the appropriate place to end the ''Hitchhiker's'' series (it was a bad CreatorBreakdown, ending with [[spoiler:all AlternateUniverse Earths destroyed and and 80% of the main cast dead.]]), and implies that ''Salmon'' would either have been retooled into the sixth part, or a {{crossover}}.
* Creator/IsaacAsimov's ''{{Foundation}}''. Several of his more well-known stories are actually one [[TheVerse universe]], through CanonWelding. He was a tremendously prolific writer, so only a small fraction of his actual work fits into the background and timeline of the Robots/Foundation [[TheVerse 'verse]], but that fraction includes some of his most popular stories. It should be noted, though, that the familiar trilogy was originally published in ''eleven'' installments[[note]]Book 1 is the first four stories, plus a framing story; Book 2 is two stories, one of which was published as a two-part serial, and book 3 is also two stories, one of which was published as a three-part serial[[/note]].
* When Bernard Cornwell was inspired by the popularity of the ''{{Sharpe}}'' television series to write some more novels, he wrote three books set prior to the Peninsular War setting of the existing novels. They were quickly dubbed the prequel trilogy by fans. Then he wrote two more. (Although as the first three concerned Sharpe's adventures in India and the other two dealt with the earlier part of the Napoleonic conflict, they seemed to have been rebranded the India trilogy.) In the end, he only moved on to other projects when he ran out of early 19th century wars for [[TheGump Richard Sharpe]] to fight in. [[TropesAreNotBad The quality of writing remained consistently good throughout.]]
* AnneRice's ''VampireChronicles''.
* MargaretWeis seems particularly prone to this:
** The ''{{Dragonlance}} Chronicles'' trilogy received a fourth book, ''Dragons of Summer Flame'', which wasn't written until after several other series and standalone works had been made in the same verse. It was written 14 years after the third book, took place 30 years after, and didn't star any of the characters from the originals (though some of them did have supporting roles). Many feel that the only reason it was dubbed part of the ''Chronicles'' series was for [[DolledUpInstallment marketing purposes]].
*** More specifically, to explain and promote a new Dragonlance setting for the DungeonsAndDragons game.
** The ''[[TheDarkswordTrilogy Darksword]]'' "trilogy" by Weis and Hickman consists of four books. The fourth is written in a somewhat different style than the rest (taking place after a TimeSkip and being narrated by a new character), but does conclude important plot threads that the third book left dangling.
*** There is also another book (Darksword Adventures) which is half novella set in the same setting as the trilogy, and half {{RPG}} system.
** Weis's solo series ''Literature/TheStarOfTheGuardians'', whose third book concluded with the main character rightfully crowned king and all the main characters getting what they deserved... Including the NominalHero, who faked his death and returned to a life of penitent obscurity in light of his now-dead OneTruePairing. Then Weis published a fourth book to clean ''them'' up. (And, even better, she then took her version of the MagnificentSeven to AscendedExtra levels by publishing a trilogy about ''them''! They're not really related, and main characters from ''Guardians'' rarely appear in ''Mag Force 7'', but, still.)
* ''{{Dune}}'' is an interesting example. ''Dune'' was actually conceived as one long book, with the sequels ''Dune Messiah'' and ''Children of Dune'' fitting directly after the first. ''Messiah'' was fleshed out while writing ''Dune'' and eventually became its own novel, which due to its expansion then warranted ''Children'' to be expanded as well and also became its own book. ''God Emperor of Dune'' and the last two in the series, ''Heretics'' and ''Chapterhouse'' are genuine examples of a trilogy creep, though the fact that the story is now over ''10,000'' years past in the originals, it's fair to say that they're a trilogy of their own. [[AuthorExistenceFailure Before he died]], Herbert planned to write a seventh book which would've been the last of a trilogy, with ''God Emperor'' serving as a bridge between the first and second trilogies. With this all said, someone "found" notes in a safe/vault/deposit box supposedly with a lot of notes on how the series was supposed to end. What happened? A prequel trilogy leading up to the original Dune novel. Followed by a pre-prequel to the whole series set in the distant past. And finally the closure of the series... followed by more books in the form of interquels(?). The canonicity is up for debate among fans.
* ''Literature/TheWheelOfTime'' was supposed to be a trilogy. Eleven books later, [[AuthorExistenceFailure the author died]]. And what was meant to be the twelfth and final book has since expanded to form its own trilogy. At World Con 2008 Tom Doherty of Tor Books finally put this long-standing rumour to rest: it was originally planned to be six books when Robert Jordan proposed the series to him in 1984, before he even started writing the first book. It's also been stated that Jordan planned for it to be three but Doherty made it a six-book deal due to Doherty's knowledge that Jordan always wrote more than he thought he needed in the first place. Jordan's original plan was for Rand to get Callandor at the end of the first book.
* ''Literature/InheritanceCycle'' also got a fourth book. The page was originally called ''The Inheritance Trilogy'' until the announcement. In this case, it's because the fourth book didn't expand the story, the third and fourth book just got too long to release as one. At 866 pages the last book is almost as long as the entire The Lord of the Rings story. The third book, Brisingr, is another 750 pages so splitting it up was a logical choice.
* ''The Literature/EarthseaTrilogy'', expanded with ''Tehanu: The Last Book of Earthsea''. Then it was renamed to ''The Earthsea Cycle''. The series expanded to six books (and one of those is a short story collection).
* The ''{{Belgariad}}'' was planned to be a trilogy ("Belgarion", "Ce'Nedra" & "Torak"). Eddings explains in ''TheRivanCodex'' that due to length and the publishing standards of large book chains at the time, his publisher convinced him to do a pentalogy instead. Then, when he was writing book four, he realised that he was going to have plenty of material left over, and the second pentalogy was planned. And when that wasn't enough, they went and planned a prequel, that in fact saw the light as two... Of two books each. ''TheElenium'', on the other hand, ''was'' a trilogy that received ''another'' trilogy of sequels.
* PiersAnthony lampshades this by marveling at how long the ''Literature/{{Xanth}}'' "trilogy" has become in the afterword to one of the books. He then mentions the possibility of working on more books in the ''Apprentice Adept'' series, since that trilogy was "looking a little sparse" at only three books. It eventually reached seven. The first Xanth "trilogy" happens to have 27 books, with the last of these being titled "Cube Route" (and since 3 cubed is 27, this is another of Piers' in/famous puns). In fact, this seems to happen to Anthony a lot. The Cluster trilogy ended up being supplemented with two {{Interquel}}s. The ''IncarnationsOfImmortality'' series, though never a trilogy, ended up three books longer than originally planned. As for the Literature/ApprenticeAdept Trilogy - it's now two trilogies and a seventh standalone.
* Mickey Zucker Reichert's ''[[TheLastOfTheRenshai Renshai]]'' trilogy started out as a stand-alone trilogy. Then she wrote a sequel trilogy. And then she wrote a sequel to the sequel trilogy.
* Mostly averted by TadWilliams; his novels are about as {{Doorstopper}} as they come, but his series last exactly the number of books that he intends them to last. However, ''To Green Angel Tower'' (of the ''MemorySorrowAndThorn'' trilogy) was split into two books for the paperback edition and at least one translation. This inadvertently happened with another trilogy of his, in that the book became so huge that it had to be split in half. It's not so much "trilogy creep" as "trilogy overflow."
* RobertRankin's ''Brentford Trilogy'' has seven books. The ''Armageddon'' series, perhaps in reference to this, is called an Octology, despite there only being three books in it.
* Creator/StephenKing declared in his afterword to ''Literature/TheGunslinger'' that he foresaw ''Franchise/TheDarkTower'' eventually comprising six to seven novels, spanning 3,000 or so pages, and taking anywhere from 20 to 300 years to write. He made good on the ''letter'' of his promise; the finished series is seven books and runs a little over 3,000 pages, but the series itself [[CanonWelding spilled over]] into over a dozen of his other novels, which amounts to tens of thousands of pages overall.
* In the foreword for ''Literature/BlandingsCastle'', Creator/PGWodehouse jokes about this by referring to the 'saga affliction' that grabs the attentions of unwary authors and forces them to think up more and more ideas for what was intended to be just one story.
* John Marsden's ''[[Literature/TheTomorrowSeries Tomorrow, When the War Began]]'' series was originally a very tightly written trilogy, that afterwards ended up seven books. There is also a post-war series as well now.
* RaymondEFeist's ''[[TheRiftwarCycle Riftwar Saga]]'' was written as a trilogy (''Magician'', ''Silverthorn'', ''A Darkness at Sethanon''). The first book was [[ExecutiveMeddling edited for size]] by the US publisher. Later, for the paperback edition the original content was restored, but the resulting text was split into two books (''Magician: Apprentice'' and ''Magician: Master''). The UK publisher still produces ''Magician'' as a single volume.
* Creator/JohnChristopher's ''Literature/TheTripods'' had a prequel added to it about 20 years after the original. This was not a bad story ''by itself'', but some felt it was a poor fit to the classic series.
* ''ASongOfIceAndFire'' was originally slated to be three books. By the end of book two, the writer realized he couldn't do it in three, and so expanded it to four. Since then, he's expanded that to six, and then splitting the fourth book in two, making it seven. If he can keep it that way, it would be fortuitous, as within the world of the books seven is an important number.
* The French translations of large fantasy cycles almost never use the words "trilogie", or "tetralogie", or whatever, even when they were used for the original. This is because the publishers smelled the cash-cow and made it an habit to split each book into two, three, or even four parts. The French version of ''ASongOfIceAndFire'' is currently a ''dodecalogy'' and this bizarro-version of the TrilogyCreep will eventually require the non-English speaking fans to shell upward of 200 euros just to get the completed series in paperback.
* JohnScalzi's ''Literature/OldMansWar" trilogy has now gained a fourth book, retelling the events of the third book from another character's POV.
* Stephenie Meyer's publishers wanted her to stop at three books about [[Literature/{{Twilight}} Bella and Edward]]. She took four books to finish out their story, and planned on writing the first book from [[PerspectiveFlip Edward's point of view]], but that was scrapped when [[CreatorBreakdown the rough draft was leaked online]].
* Lynn Flewelling's ''Literature/{{Nightrunner}}'' series gained a fourth book years after the first three were originally published, but this wasn't a change of plan: book three had an author's note explicitly stating that "This is not a fantasy trilogy; it's a series that happens at the moment to be three books long".
* David Gerrold originally planned for his ''Literature/TheWarAgainstTheChtorr'' series to be a trilogy, before he realized he was going to need more room. It's currently up to four books.
* Creator/RobertLudlum's ''Literature/TheBourneSeries'' has a clear ending in book three, as Bourne/Webb finally kills his nemesis Carlos the Jackal. After Ludlum's death Eric van Lustbader continued the series. Interestingly [[Film/TheBourneSeries the film adaptations of the books]], which bear only the most superficial resemblance to their source material, also provide a definite ending in the third entry. But then they started making ''Bourne 4''.
* Anne Bishop's [[BlackJewels The Black Jewels Trilogy]] has six books (and a short story/novella) collection. Averted in that the first three books are a coherent trilogy (all with titles Noun of the Noun), followed by a prequel, followed by the collection which has prequel, inter, and epilogue stories, followed by two epilogue novels.
* MaximumRide suffers from a very sad case of this -- the first book held up fine as a solo work, and the next two sketched out a decent story arc. The fourth book released was received much less positively in comparison.
* Diana Gabaldon's ''Literature/{{Outlander}}'' was originally supposed to be two books, then three, then four, then a double-trilogy. The seventh book came out in Fall 2009. The author never explicitly said to her publishers that it was ''supposed'' to be a trilogy-- only that she had at least two more books after the first in her, and she ran with that. Several of the Lord John stories exhibit short story creep, since Gabaldon's idea of a [[{{Doorstopper}} short]] story grew into a publisher's idea of a complete novel.
* HarryTurtledove, particularly since it was unexpected: his [[Literature/{{Timeline191}} TL-191]] series started with a one-off prequel, ''How Few Remain'', then a trilogy called ''The Great War''. All of which fitted the planned releases that had been "Coming Soon" in the fronts of his novels for years. Then suddenly the one round-up book that would have dealt with events later on, called ''The Great War: Settling Accounts'', grew to '''seven huge books''', the ''American Empire'' trilogy and then the ''Settling Accounts'' tetralogy. The vast amount of padding and repetition involved in these seven, along with what is broadly considered to be a significant decline in writing quality, has led [[AlternateHistoryDotCom some]] to accuse Turtledove of deliberately writing {{Doorstopper}}s to put his kids through college.
* Jane Yolen's ''Pit Dragon Trilogy'' became the ''Literature/PitDragonChronicles'' when a fourth book was released... [[SequelGap over two decades after the trilogy "ended."]]
* ''Literature/TheEmigrants'' by Vilhelm Moberg was originally intended as a trilogy, but the third book was split into two. It is even more noticable in the original Swedish titles, where the fourth book has an OddNameOut (''The Emigrants'' about leaving Sweden, ''The Immigrants'' about arriving in America and traveling to where land is handed out, ''The Settlers'' about starting up farming in a new country - and ''The Last Letter Home'' continues that story). The second book is normally called ''Unto a Good Land'' in English.
* Scott Westerfeld dedicated "[[Literature/{{Uglies}} Extras]]" to "everyone who wrote to me to reveal the secret definition of the word 'trilogy'". Proving TropesAreNotBad, "Extras" is essentially a companion piece with some trilogy characters as {{Special Guest}}s.
* Chris Walley's ''Literature/LambAmongTheStars'' trilogy is actually an {{aversion}} in that when the third book was written and found to be a {{Doorstopper}}, Walley went back and edited the first two books into a single book. The new third book ends the series in such a way that it would be incredibly difficult to continue it. This was a case of subverted ExecutiveMeddling, since Tyndale asked him to do it. Subverted in that having three similar-sized books works much better than two small and then two large.
* Cassandra Clare's ''MortalInstruments'' books provide a rather outrageous example. First, there was the original trilogy (''City of Bones'', ''City of Ashes'', and ''City of Glass''). Then, it was announced that Clare was writing a steampunk prequel trilogy set in Victorian London (''The Clockwork Angel'', ''The Clockwork Prince'', and ''The Clockwork Princess''). ''Then'', a fourth book centered around the Simon character of the first trilogy was revealed to be in the works (''City of Fallen Angels''). '''''Then''''' Clare decided to add two more books to the MortalInstruments story (''City of Lost Souls'' and ''City of Heavenly Fire'') while also stating that this new trilogy of MI books would no longer focus solely on Simon but rather on the entire cast. This YA fantasy book series literally tripled in size. Almost humorously, after writing the third book Clare stated in an interview that she liked half-open endings, and deliberately left hers that way, and that she had no intention of continuing it.
* TheIcemarkChronicles was originally supposed to be a trilogy, but the author has announced he intends on writing a fourth, set before the trilogy.
* The first three books of [[Literature/{{Dragons}} The Last Dragon Chronicles]]form a definite trilogy, with a complete story, and a ''very'' definite ending. The rest do continue it, but begin a new story arc altogether.
* The SecretHistories series was originally planned to be a trilogy, but the series proved so popular that SimonRGreen decided to make it into an ongoing series.
* Creator/JohnRingo's ''Literature/TroyRising'' series was originally planned to be a trilogy, but word on the Ringo forum on [[Creator/BaenBooks Baen's Bar]] is that his Muse is insisting on continuing the series, much to the joy of many of his readers. The current plan is for five books total, unless Ringo's Muse insists on more.
* Meg Cabot's ''PrincessDiaries'' series was planned to be a trilogy, and the first three books do make for a complete story, but Cabot kept writing books until they reached the number of ten, not counting spin-offs.
* The "The New Prophecy" arc in ''Literature/WarriorCats'' was conceived of as a spin-off trilogy, but turned into a six-book sequel series.
* ''Mattimeo'', the third book in the ''{{Literature/Redwall}}'' series, ended on a note that was clearly supposed to be a wrap-up. Trouble is, author Brian Jacques got addicted to the universe he'd been writing in and had written 22 books (plus additional material) by the time he died.
* ''[[Literature/TheChroniclesOfNarnia The Lion, The Witch, and the Wardrobe]]'' was originally intended to be a single, self-contained story. Then Lewis decided to write a sequel. Then he wrote a third book, and made sure to make it very "final": the characters sail over the edge of the world and find themselves in Aslan's (IE God's) country, the two protagonists remaining from the first book are told that they will not be returning to Narnia, and Aslan reveals to them his full glory. He ended up writing more books, but stopped at seven, deciding it was a good number to end on.
* Just before the third book of the ''Literature/HushHush'' trilogy came out, the author announced that she would be writing a fourth book too.
* SergeyLukyanenko originally just wrote ''Literature/NightWatch'' as a single novel. Following its success (in Russian-speaking countries), he continud with ''Day Watch'' and ''Twilight Watch'' with the co-author of ''Day Watch'' writing a spin-off featuring none of the main characters. Then followed ''Final Watch''... and ''New Watch''. Many fans agree that the series has long ago [[JumpingTheShark jumped the shark]]. To try to remedy this the author turned the series into a SharedUniverse, inviting younger authors and promising strict quality control over what gets published. As of summer 2014 fans' reaction to new collaborations is yet mixed and undecided.
* Creator/SpikeMilligan's war memoirs started with ''Literature/AdolfHitlerMyPartInHisDownfall'' and were planned as a trilogy. Eventually, there were seven books, extending out significantly beyond the actual war.
* Averted with ''[[TheLightbringerTrilogy the Lightbringer Series]]''. The author intended for it to be a trilogy, but purposefully named it "Series" just in case he went over. And indeed, he later started with a fourth.
* Brian Lumley's ''{{Necroscope}}'' series was supposed to be a trilogy, and book three does have a fairly solid BittersweetEnding, but then he figured it was too much of a downer and wrote two more books which ended the series on another bittersweet ending. This was expanded by another three books (''Necroscope: Vampire World'') providing ''another'' bittersweet ending to the series. Which was then continued in a pair of {{Interquel}} works which ended on a flat out DownerEnding just to shake things up. Finally another three books followed in which, not only was another bittersweet ending, but also with Lumley deciding to TorchTheFranchiseAndRun. Which closed out the former trilogy on book ''thirteen''. Except for the novella, short stories, and another {{Interquel}} book he later wrote.
* S. Andrew Swann's Literature/MoreauSeries had a fourth book, ''Fearful Symmetries'', added on five years after the original trilogy was completed.
* Lois Lowry's ''TheGiver'' (1993) had two sequels, ''Gathering Blue'' (2000) and ''The Messenger'' (2004), and was fittingly referred to as "the ''Giver'' trilogy" for eight years. Now it's been announced that a fourth book, ''The Son'', is in the works, due to be released in 2012 or 2013.
* ''TheBartimaeusTrilogy'': It was a trilogy, then along came the announcement of a prequel. The page is still called "The Bartimaeus Trilogy", even after the release of the prequel. This is reasonably fair, though, as the prequel book is very much a self-contained story and only features two characters seen in the original novels: Bartimaeus himself and his long-time antagonist, Faquarl.
* An odd case in the case of the GreenSkyTrilogy. The first book set up the story, the second was more or less a PerspectiveFlip from Teera's POV, the third dealt with the fallout of the events on the first two books. But...Snyder decided to play DeathByNewberyMedal on a lead character and realized she shot herself in the foot. Then, a software designer shows up, wanting to make an adaptation of her books. Queue what was perhaps the first video game to be called a {{canon}} sequel.
* Inverted with ''Literature/TheLordOfTheRings'': when Tolkien finished it, it was in six 'books', with Tolkien wanting it published either all at once or possibly in two volumes. His publishers split it into a trilogy.
* Early editions of ''[[Literature/PercyJacksonAndTheOlympians The Lightning Thief]]'' and ''The Sea of Monsters'' had "Percy Jackson & the Olympians '''Trilogy'''" written on their spines. There ended up being 5 books in the series.
* Philip Pullman's ''[[Literature/HisDarkMaterials His Dark Materials]]'' series was originally a trilogy. It was followed a few years later with two novellas, Lyra's Oxford and Once Upon a Time in the North, and another full novel, The Book of Dust, is expected... [[SequelGap eventually]] (as of this writing, almost twenty years have passed since the publication of the first book).
* For the first two books, the ''Literature/TheLostYearsOfMerlin'' series by T.A. Barron was advertised as a trilogy. With the third book, it was decided that two more would follow and the billing for the series became an "epic" instead. In the introduction for the third book, the author states that Merlin himself told him that three books would not be enough. This isn't quite so galling, though, when you consider that in the introduction to the first book, he states that the story came to him through Merlin.
* When the first novel of ''Literature/WindAndSparks'' cycle was published, Alexey Pehov said that the story got too long for a single book, but he'll wrap it up in the second one. Then in the third. It ended up as four novels and two prequel short stories. The irony? It was inspired by ''Literature/TheWheelOfTime'' (but less epic, a bit darker, much snarkier), with a helping of ''VideoGame/{{Thief}}'' and ''Literature/GarrettPI''.
[[/folder]]

[[folder:Live-Action TV]]
* ''Series/KamenRiderDenO'''s third movie was explicitly called "Farewell Kamen Rider Den-O: The Final Countdown", and features a PassingTheTorch aspect with the introduction of a new Den-O, and on a meta level it was believed to be the end of the series because star Takeru Satoh was moving on to other roles. Another three / five ''Den-O'' movies[[note]]depending on whether you count the Chō Den-O Trilogy movies as one big one or three smaller ones[[/note]] have come out since then. Possibly lampshaded in the DVD release of "Final Countdown", where a short extra cartoon has one character remark that, for all the talk of "final" and "conclusion", that doesn't stop them from making more sequels.
[[/folder]]

[[folder:Music]]
* The {{Rush}} song "Where's My Thing?" is described as part IV of the Gangster of Boats trilogy. [[{{Uninstallment}} There kinda actually wasn't any Gangster of Boats songs before it.]]
** Played straight with ''Vapor Trails''' "Freeze (Part IV of Fear)." Fear is a "trilogy" that was [[AnachronicOrder released in reverse order]] with Part III, "Witch Hunt," from ''Moving Pictures'' first and Part I, "The Enemy Within" from ''Grace Under Pressure'' released third.
* Played straight AND subverted by BadReligion, where Part III received a sequel and a prequel on the album 'Suffer' in Part II (The Numbers Game) and Part IV (The Index Fossil), but no Part I. [[WordOfGod Brett Gurewitz, when asked by a fansite, cited Big Bang as the closest to Part I that he could think of.]]
[[/folder]]

[[folder:Theatre]]
* Though ''Theatre/DerRingDesNibelungen'' was not originally conceived as a trilogy, it was already four plays by the time Creator/RichardWagner began composing the music, and is not commonly thought of as a trilogy, its official heading is "a stage festival play for three days and one evening before."
* It is believed by some scholars that Creator/WilliamShakespeare wrote the three ''Theatre/HenryVI'' plays out of sequence: first [[Theatre/HenryVIPart2 Part II]], then [[Theatre/HenryVIPart3 Part III]], then finally [[Theatre/HenryVIPart1 Part I]]. Moreover, from a critical and performing standpoint, they are often lumped together with ''Theatre/RichardIII'' as an overall "UsefulNotes/WarsOfTheRoses" story.
[[/folder]]

[[folder:Video Games]]
* ''Franchise/{{Halo}}'' is an interesting case, as it was originally planned to be only '''[[TwoPartTrilogy two]]''' games; [[ExecutiveMeddling Microsoft wanted a third game]] and development time for ''Halo 2'' ran short, so they stretched it out over ''VideoGame/{{Halo 2}}'' and ''VideoGame/{{Halo 3}}''. Then there's ''VideoGame/Halo3ODST'' (planned, but expanded) and ''VideoGame/HaloReach'' (never planned), which roughly fit into the same "trilogy". In 2012, ''VideoGame/{{Halo 4}}'' was released, which is the beginning of a second ''Halo'' trilogy.
* ''VisualNovel/ApolloJusticeAceAttorney'', the fourth game in the ''VisualNovel/AceAttorney'' series. Shu Takumi had planned the series to be a trilogy, but Capcom called for another game. The protagonist change was due to the fact that the former three games already told all the stories for Phoenix.
** Likewise, ''VisualNovel/{{Ace Attorney Investigations|MilesEdgeworth}}'' was meant to be a GaidenGame, with a fifth game in the main series planned. ''Investigations'' ended up getting its own sequel before ''5'' was released, though ''Investigations 2'' was [[NoExportForYou Japan-only]].
* Both "''Hitman'' Trilogy", a boxset for the [=PS2=], and "''Hitman'' HD Trilogy", a similar set for the [=PS3=] and 360, actually feature the second, third and fourth entries in the ''VideoGame/{{Hitman}}'' series (''Silent Assassin'', ''Contracts'' and ''Blood Money''), skipping the original game in the entirely. The original game in the series, ''Hitman: Agent 47'', was a PC-exclusive; not to mention ''Contracts'' is, in effect, a remake of it.
* Creator/HideoKojima did not intend to direct any of the ''Franchise/MetalGear'' sequels beyond ''VideoGame/MetalGearSolid''. When he was brought back to direct ''VideoGame/{{Metal Gear Solid 2|SonsOfLiberty}}'', he ended the story with an inconclusive ending and then followed it up with ''VideoGame/{{Metal Gear Solid 3|SnakeEater}}'', a prequel that was marketed as "the final game in the Metal Gear Solid trilogy." However, due to much demand after the open-ended nature of ''Sons of Liberty'', Kojima was brought back again for ''VideoGame/{{Metal Gear Solid 4|GunsOfThePatriots}}'' to tie up all the loose ends and conclude Solid Snake's storyline. Despite this, the Metal Gear series continues on with a series of prequels starring Big Boss (such as ''VideoGame/MetalGearSolidPortableOps'' and ''VideoGame/MetalGearSolidPeaceWalker'') and ''VideoGame/MetalGearRisingRevengeance'' (a spin-off starring Raiden). Then Kojima started directing ''[[VideoGame/MetalGearSolidVThePhantomPain Metal Gear Solid V: The Phantom Pain]]'', which he is claiming to be the final ''Metal Gear'' game he is directly working on.
* After the release of ''VideoGame/LeisureSuitLarry 3: Passionate Patti in Pursuit of the Pulsating Pectorals'', series creator Al Lowe insisted that there wouldn't be a ''Larry [[ExactWords 4]]'', partly thanks to how Lowe essentially wrote himself into a corner without any idea how Larry 3's ending could lead into a sequel. Instead, he chose to skip the fourth installment (retroactively referred to as ''Leisure Suit Larry 4: The Missing Floppies'') completely and carry on straight into ''Leisure Suit Larry 5: Passionate Patti Does a Little Undercover Work'', where the absence of a fourth game is actually a ''plot point'', and both Larry and Patti have amnesia because the BigBad stole the Larry 4 disks.
* ''VideoGame/JakAndDaxter'' was originally intended as a trilogy and thus was written as such: at the end of the third one the Precursors are revealed and the BigBad is defeated. However, a racing spinoff was released a year later, followed by a PSP game with Daxter in the lead, and a [=PS2=] game developed by a different team than original creator Naughty Dog.
* After ''VideoGame/PrinceOfPersiaTheTwoThrones'' wrapped up the trilogy started with ''[[VideoGame/PrinceOfPersiaTheSandsOfTime Sands of Time]]'' and continued with ''[[VideoGame/PrinceOfPersiaWarriorWithin Warrior Within]]'', a fourth game in the same continuity, titled ''VideoGame/PrinceOfPersiaTheForgottenSands'', was made in order to capitalize on the release of the ''[[Film/PrinceOfPersiaTheSandsOfTime Sands of Time]]'' movie. A ContinuityReboot had been produced in the intervening years.
* After ''VideoGame/AnotherCenturysEpisode 3: The Final'' came out, many people were hoping for a new "A.C.E." game would be released sometime. Low and behold, ''Another Century's Episode: R'' for the PlayStation3. Note, however, that it is more or less a new continuity as opposed to an actual story continuation. Incidentally, [[WordOfGod Director Terada more or less said]], "I don't know why we called ''A.C.E.3: The Final'' (due to them apparently having every intention of making a new game). So, let's just say that ''A.C.E.3'' was "The Final PlayStation2 ''A.C.E.'' game."
* When ''Franchise/{{Fabula Nova Crystallis|FinalFantasy}}'' was conceived, it was originally meant to consist of [[VideoGame/FinalFantasyXIII three]] [[VideoGame/FinalFantasyXV self-contained]] [[VideoGame/{{Final Fantasy Type-0}} games]] that shared one mythology. Nine years later, and it jumped to six games and about three or four short stories, with possibly more coming along the way.
* The ''VideoGame/{{Onimusha}}'' series was originally conceived as a trilogy, with ''Onimusha 3: Demon Siege'' initially advertised as the final installment of the series. It didn't take long for Capcom to produce a fourth game afterward, with ''Onimusha: Dawn of Dreams'' (aka ''Shin Onimusha'').
* There are rumors that {{Creator/Bioware}} is working on sequels to the VideoGame/MassEffect trilogy. The galaxy was saved, so it's unclear what storyline it would follow.
[[/folder]]

[[folder:Web Animation]]
* WebAnimation/TheLazerCollection was originally only going to be three parts long, but when requests for a fourth part were continuously made, [[http://www.youtube.com/watch?v=8aLIP94KBlw&feature=channel the author eventually made it]], but it's a case of StylisticSuck with a subtext of "[[ArtistDisillusionment Now Shut Up]]."
** There's also a [[http://www.youtube.com/watch?v=Ws4UVguoZjM&feature=feedu Part Five]].
[[/folder]]

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