Acrossover is the placement of two or more otherwise discretefictional characters,settings, oruniverses into the context of a singlestory. They can arise from legal agreements between the relevantcopyright holders (known asintercompany crossovers), common corporate ownership or unofficial efforts byfans.

This is different from aspoof, where one discrete character, setting, or universe, copies another character, setting, or universe, often in a comedic manner.

Crossovers often occur in an official capacity in order for theintellectual property rights holders to reap the financial reward of combining two or more popular, established properties. In other cases, the crossover can serve to introduce a new concept derivative of an older one. Another intention is to give fictional characters more emotional credibility and thus increaseimmersion for the fans.

Crossovers generally occur between properties owned by a single holder, but they can, more rarely, involve properties from different holders, provided that the inherent legal obstacles can be overcome. They may also involve using characters that have passed into thepublic domain with those concurrently undercopyright protection.

A crossover story may try to explain its own reason for the crossover, such as characters being neighbors (notable examples being the casts fromThe Golden Girls andEmpty Nest) or meeting via dimensional rift or similar phenomenon (a common explanation forscience fiction properties that have different owners). Some crossovers are not explained at all. Others are absurd or simply impossible within the fictional setting, and have to be ignored by the series' respectivecontinuities. Still others intentionally make the relations between two or more fictional universes confusing, as withThe Simpsons andFuturama, where each show is fiction in the other.

In contrast with legal crossovers, unofficial crossovers are created solely because of the artistic pleasure derived by their creators. Unofficial crossovers often take the form offan-written fiction^[1] andfan art, but the trope is increasingly prevalent inamateur films andaudio. Whereas official crossovers are frequently stymied by such concerns as copyright, royalties payments, quality of writing and ownership of the characters, unofficial crossovers are unfettered by such concerns, so long as property holders do not exercise their right toenjoin the distribution of such material. A good example would be the unauthorised live action fan filmBatman: Dead End which brings together the properties ofBatman,Alien andPredator in one setting.

Unofficial crossovers can also occur in a "what-if" scenario.Roger makes frequent cameo appearances inFamily Guy, whileBrian makes cameos onAmerican Dad!. Roger,Rallo Tubbs, andKlaus Heissler were seen in the finalFamily GuyStar Wars spoof, "It's A Trap!", asMoff Jerjerrod,Nien Nunb, andAdmiral Ackbar, respectively. Stewie also appears as an interactive hallucination of Booth onBones when the agent has issues over possibly becoming a sperm donor, with David Boreanaz (who plays Booth) repaying the favor in "Road to the North Pole". An appearance byElmo, fromSesame Street, was made, in a hallucination ofConnie Ray's, on TV sitcomThe Torkelsons. Fan fiction fusions between different science fiction movies and series are often created, such asStar Wars andStar Trek orBabylon 5 andStargate.M.U.G.E.N. is a fighting game engine that features many fan-created and fictional characters and stages from various television series, movies, as well as other video games.

Crossovers of multiple characters, owned by one company orpublished by onepublisher, have been used to set an established continuity, where characters can frequently meet within one setting. This is especially true ofcomic book publishers, as different characters in variousMarvel,DC, orValiantcomic books frequently interact with one another since they live in a "shared universe". For example, in the Marvel Comics universe,Spider-Man has frequent dealings with another Marvel hero,Daredevil, just as in the DC Comics Universe, theFlash andGreen Lantern often collaborate. In comic book terminology, these "guest star" roles are common enough that they are generally not considered crossovers; rather, this short-term collaboration to fight crime is called ateam-up. A crossover in comic book terms only occurs when a story spans more than one title.^{[citation needed]} This has led to "crossover events" in which major occurrences are shown as affecting most or all of the stories in the shared universe; seeCategory:Crossover comics.

The earliest such crossover event wasGardner Fox'sZatanna's Search which took place inHawkman #4 (October/November 1964),Detective Comics #336 (February 1965),The Atom #19 (June/July 1965),Green Lantern #42 (January 1966),Detective Comics #355 (September 1966), andJustice League of America #51 (February 1967). This story dealt withZatanna attempting to reconnect with her father,Zatara, and seeking the aid of Hawkman, Batman,Robin, the Atom, Green Lantern, and theElongated Man along the way.

The first major crossover event was spearheaded by the Marvel Editor-in-Chief at the time,Jim Shooter. As a way to further toy sales he devised theSecret Wars crossover which brought all the major Marvel heroes into a 12-issue miniseries to battle a common threat. After the threat was dealt with, they all returned to their regular titles.Secret Wars was hailed as both a critical and commercial success, largely because the events of the crossover had lasting effects on the characters (such as the introduction ofSpider-Man's black suit which would later become the villainVenom). Jim Shooter later perfected his crossover technique atValiant Comics with theUnity event.Unity brought all the Valiant characters together to defeatMothergod, but was told within the existing Valiant Comics titles (and two bookend special issues). Readers were not obliged to buy all 18 chapters as the story was coherent when reading just one title, but far more layered when all were read. LikeSecret Wars, theUnity crossover had lasting effects on the Valiant universe; most notably the introduction ofTurok, the birth ofMagnus, Robot Fighter and the death of a major Valiant hero.

Cartoon crossovers are not uncommon, and most of them – like comics or live-action TV shows – will often feature characters owned by the same company or network. One example isCartoon Network'sThe Grim Adventures of the KND. It features five crossovers –Ed, Edd n Eddy,Codename: Kids Next Door,The Grim Adventures of Billy & Mandy, a reference toThe Powerpuff Girls, and a quote fromScooby-Doo, which are all licensed Cartoon Network series.During the 1970s and 1980s, crossovers were particularly common among theHanna-Barbera properties. Some of the earliest examples happened onThe New Scooby-Doo Movies which featured appearances by characters fromHarlem Globetrotters,Josie and the Pussycats,Jeannie,Speed Buggy,Batman and Robin, andThe Addams Family. Later, theHanna-Barbera Superstars 10 set of movies involved several crossovers, including such combinations asThe Jetsons Meet the Flintstones. This was taken to an extreme in the 1977–79 seriesLaff-A-Lympics, which was essentially a gathering of the Hanna-Barbera characters for a regular series.

Crossovers are not necessarily composed of characters under common ownership. Two of the most notable cartoon crossovers consisted of characters from different companies. Disney's movieWho Framed Roger Rabbit had characters from various companies, most notably Disney andWarner Bros. The film also includes cameos of characters fromMGM.

Another cartoon crossover would occur in 1990,Cartoon All-Stars to the Rescue. This cartoon featured popular characters from children's Saturday morning cartoons, banding together to promote an anti-drug message.ABC,CBS,Fox, andNBC aired this half-hour special one Saturday morning with characters from all their networks, includingHuey, Dewey, and Louie (from Disney'sDuckTales),Winnie the Pooh,Tigger, Slimer (fromThe Real Ghostbusters andExtreme Ghostbusters), Michelangelo (fromTeenage Mutant Ninja Turtles), Bugs Bunny, Daffy Duck,Alvin and the Chipmunks,the Smurfs, ALF (fromhis short-lived cartoon spinoff),Garfield, and the trio ofBaby Kermit,Baby Piggy, andBaby Gonzo (from Jim Henson'sMuppet Babies). Animation companies granted unlimited, royalty-free use of their cartoon characters for this project, a feat that has been unequalled before or since. This cartoon was also introduced by then-PresidentGeorge H. W. Bush andBarbara Bush, and would be distributed to schools and video stores free of charge nationwide.

Webtoons (animated shows from the internet) can also have crossovers with different webtoons, franchises, YouTubers and more. Some examples are when Kate fromTomSka'sCrash Zoom series makes an appearance inEddsworld as a trick or treater who possesses Matt and Edd in the episode, "Trick or Threat". In addition to this a couple of characters fromEddsworld make appearances in someCrash Zoom episodes such as "Orcs and Dorks".

Anime has also participated in many crossover events featuring characters or shows from the same company or network. One of the biggest projects down would beDream 9 Toriko x One Piece x Dragon Ball Z Super Special Collaboration as it includes threeShonen Jump franchises, beingDragon Ball Z,One Piece, andToriko, crossed over into an hour long special-like most crossovers, this special is filler, a fan-service episode that follows the common plot line in most crossovers. What makes this crossover unique is when the characters from all three shows split into groups, where the members all share the same clichéd character archetypes, such the main characters Goku, Luffy, and Toriko falling into the dumb, good-natured, strong character archetype.^[2]

Manga artistLeiji Matsumoto has been known to cross over the characters of his various stories and characters such asCaptain Harlock,Galaxy Express 999, andQueen Millennia, all of which were originally written as separate, self-contained stories. In theMaetel Legend, Queen Promethium is revealed to be having been Yukino Yaoi, the protagonist fromQueen Millennia. Matsumoto has also created various crossovers withSpace Battleship Yamato, an anime on which he served as director, although the rights to Yamato are actually owned by Yoshinobu Nishizaki.

The first film crossover in a series ofUniversal Studios monster films wasFrankenstein Meets the Wolf Man, in 1943. After the comics publishing houseMarvel Comics ventured intomovie production, they set off to produce solitary films with popular superheroes from the Avengers team, with characters from upcoming films making cameo appearances in films starring another superhero, leading up to the crossover filmThe Avengers (2012). Inspired by Marvel's success,Warner Bros., who hold movie rights forDC Comics' heroes, announced the production ofBatman v Superman: Dawn of Justice (2016) and further plans to develop the cinematicDC Extended Universe, whileParamount Pictures andEntertainment One announced plans to create a cinematic universe on theTransformers film series.

There have been numerous crossovers in Japanese cinema. The boom ofkaiju films saw a lot of crossovers produced atToho Studios, with some of the monsters forming teams in numerous movies, much like the Marvel movie franchise.Godzilla,Mothra andRodan (Radon) each first appeared in standalone films before being set against each other or even teaming up against stronger enemies. Their first encounter was in 1964, first inMothra vs. Godzilla and a few months later of all three inGhidorah, the Three-Headed Monster. In 1962,Toho releasedKing Kong vs. Godzilla.

Crossovers in video games occur when otherwise separated fictional characters,stories,settings,universes, ormedia in avideo game meet and interact with each other. These can range from a character simply appearing as aplayable character orboss in the game, as a special guest character, or a major crossover where two or more franchises encounter.

Konami made the first crossover video game featuringSimon Belmont fromCastlevania,Universal Pictures'King Kong and Mikey from the Warner Bros. movieThe Goonies inKonami Wai Wai World for the Famicom in 1988.

Mario & Sonic at the Olympic Games, released in Japan two months beforeSuper Smash Bros. Brawl, was the first time thatMario and Sonic (as well as their associated characters) appeared in a game together.

Inliterature, some authors also engage in crossovers by including characters from different novels they have written in one.

The first popular crossover in literature was the 1885Mark Twain novel,The Adventures of Huckleberry Finn, which had an important guest appearance byTom Sawyer. Similarly, Lady Glencora Palliser from the Pallisers series ofAnthony Trollope appears towards the end ofMiss Mackenzie, a novel published between the first and second Palliser novels in 1865, a character first introduced in the novel,Can You Forgive Her? (1864).Andrew Lang's 1890 collection,Old Friends: Essays in Epistolary Parody, contains letters combining characters from different sources, including one based onJane Austen'sNorthanger Abbey andCharlotte Brontë'sJane Eyre.^[3]

Kim Newman frequently uses this device, as doesStephen King. The works ofJames Branch Cabell,J.D. Salinger,William Faulkner,Margaret Laurence,Thomas Pynchon,Kurt Vonnegut,Mordecai Richler,Sir H. Rider Haggard,Edgar Rice Burroughs,Robert Heinlein, andIsaac Asimov also cross over with each other, linking different characters and settings together over a number of different works.

IllustratorHoward Pyle conceived his workTwilight Land as one such crossover. In it, a nameless narrator is transported to "Twilight Land" and meets famous fairy tale characters for a soirée in an inn:Mother Goose,Cinderella,Fortunatus,Sinbad the Sailor,Aladdin,Boots, theValiant Little Tailor, and others gather in the framing device and tell each other adventurous tales featuring other literary personages.^[4]

French authorJules Lemaître wrote a sort of sequel toCinderella, namedPrincess Mimi, where Cinderella's daughter is courted byPolyphemus andCharles Perrault'sHop-o'-My-Thumb.^[5][6]

It is also common for authors to 'crossover' characters who have passed into the public domain, and thus do not require copyright or royalty payments for their use in other works.The League of Extraordinary Gentlemen byAlan Moore andKevin O'Neill is another example of this, as all of the main characters and most of the secondary / background characters are fictional characters whose copyright has expired, and all are characters of different authors and creators brought together within one massive extended universe. Many of the works ofPhilip José Farmer'sWold Newton family sequences (which has also been explored and developed by other authors) also utilize and interweave numerous otherwise unrelated fictional characters into a rich family history by speculating familial connections between them (such as a blood-relationship betweenSherlock Holmes andTarzan).Roger Zelazny's novelA Night in the Lonesome October combines Sherlock Holmes, Doctor Frankenstein, Jack the Ripper, and the Cthulhu Mythos, although he never specifically identifies them as such ("The Count", "The Good Doctor", "Jack", etc.).

Occasionally, authors will include into crossovers classic fictional characters whose copyright is still held by the original authors (or at least their estates), but who are nevertheless considered iconic or 'mythic' enough to be recognised from a few character traits or descriptions without being directly named (thus not requiring royalties payments to be made to the copyright holder). A prominent example occurs withinThe League of Extraordinary Gentlemen, Volume One, wherein a character who is clearly intended in appearance and description by other characters to be Dr.Fu Manchu appears as a significant villain; however, as this character was not in the public domain at the time of writing and the rights still held by the estate of his creatorSax Rohmer, he is not directly named as such in the work and is only referred to as 'the Devil Doctor'.

Crossovers involving principals can also occur when the characters have no prior relationship, but are related by time period, locale or profession. TheLaw and Order series, for example, afford a commonality of setting and profession which lends itself to crossovers, both within the franchise and in a wider universe. Following the cancellation of theABCsoap operaOne Life to Live and its high-rated finale, several characters crossed over into the network's remaining soap operaGeneral Hospital, remaining in the same timeline as their former show.

Though most common on shows of the sameproduction company (see, for example, "Hurricane Saturday"), crossovers have also occurred because shows share the same distributor ortelevision network. A notable example of this kind of link is that betweenMurder, She Wrote andMagnum, P.I. These shows were made by different companies, but owned byUniversal Studios and broadcast onCBS. Another case is that ofMad About You andFriends, which share the character ofUrsula Buffay. Neither show shares any production or distribution commonality, but rather an actress (Lisa Kudrow), a setting (New York City) and a schedule (Friends initially followedMad About You onNBC'sThursday night schedule).

Mad About You andFriends share another type of "network crossover". On rare occasions, networks have chosen to theme an entire night's programming around a crossover "event". In one case, a New York Cityblackout caused byPaul Reiser's character onMad About You was experienced by the characters onFriends andMadman of the People.^[7] Such "event nights" can also be linked by a single character's quest across multiple shows on the same evening.ABC attempted this kind of "event night" crossover with itsFriday night programming during the 1997 season. There, they proposed that the title character ofSabrina the Teenage Witch should chase her cat,Salem, throughBoy Meets World,You Wish andTeen Angel because it had run away with a "time ball" that was displacing each show through time.^[8]

In 2013, the Canadian crime drama seriesRepublic of Doyle andMurdoch Mysteries produced a crossover,^[9] which was complicated by the shows' incompatible historical settings;Murdoch Mysteries is a historical series set in the 1890s, whileRepublic of Doyle is set in the present day. The problem was solved by having the actors cross over asrelatives of their primary characters;Allan Hawco appeared on the November 25, 2013 episode ofMurdoch Mysteries as Jacob Doyle, a 19th-century ancestor of his regular character Jake Doyle, whileYannick Bisson appeared on a January 2014 episode ofRepublic of Doyle as Bill Murdoch, a 21st-century descendant of his regular character William Murdoch.^[9]

The earliest example of a crossover in children's television wasPBS' 1971 programThe Electric Company.

Crossovers can take the form of a promotionalcameo appearance, used to draw attention to another work of fiction, with little rational explanation in the context of the hosting show's narrative. When not clearly presented as parody, this is frequently scorned by fans as blatant commercialism. A notable example of this isThe Simpsons episode "A Star Is Burns", in which the character of Jay Sherman (fromThe Critic) appeared. It originally aired on March 5, 1995, on FOX right beforeThe Critic began its second season, its first season having aired on ABC. This episode was largely condemned by fans ofThe Simpsons as existing to promoteThe Critic, an animated series considered inferior by comparison. EvenSimpsons creatorMatt Groening objected, preferring to remove his name from the credits of that particular episode in protest.^[10]

In its simplest and most common form, a television crossover involves a starring character on a parent show appearing on aspin-off or vice versa because of established character relationships. An obvious example of this type of crossover occurred whenCliff Huxtable ofThe Cosby Show visited his daughter,Denise, onA Different World.

More complex multi-production franchises can utilize crossovers of characters to serve as a device in establishing continuity in a shared fictional universe. This crossover is common in theStar Trek universe, where minor guest stars from one series have appeared as featured guest stars later ones. A good example of this crossover is that of theKlingonsKor,Koloth, andKang. After the passage of about a century of narrative time, the three onetime adversaries ofCaptain Kirk appeared together in theStar Trek: Deep Space Nine episode, "Blood Oath" – as the Klingons and Federation had become allies in the century between, the former villains are now portrayed as heroes.

The distinction between "spin-off" and "crossover" is sometimes narrow. The two terms can become especially conflated if two shows are linked by a guest star with a single appearance. There is debate, for instance over whetherOut of the Blue is a spin-off ofHappy Days, or whether the star ofOut of the Blue merely crossed over intoHappy Days.

Often, the problems of bringing together two shows with different narrative ambitions make the writing of a crossover burdensome. Such difficulties are encountered bysituation comedies that wish to crossover withdramatic television programs. The satirical crossover—ranging in length from a cameo to a fullcomedy sketch or episode—is an extremely popular way of circumventing this problem. By various means, such crossovers typically avoid outcry from fans by being obviousparody orhomage. However, on rare occasion, the humor of such crossovers can be used by one show make anarrative point by capitalizing on the audience's experience of the other program.

Parodic crossovers can be directly established as being outside the continuity of one or all of the properties being crossed over. A good example is the crossover betweenThe Simpsons andThe X-Files, which was largely accepted as being outside standardX-Files continuity.

They can occur by virtue of adream sequence, in which the characters of one show will appear as part of a dream had by a character on another show. This method was perhaps used most famously to explain to audiences that the entirety ofNewhart had been the dream ofBob Newhart's character onThe Bob Newhart Show. It has more recently been used to demonstrate that cast members ofThe Young and the Restless appeared in a dream of a character onThe King of Queens.

Parodic crossovers can take the form of "gag" cameos by characters of one property appearing on another. Crossovers of this type can also be completely wordless. This type of crossover is more common on animated programs, such as whenBender found and ateBart Simpson's shorts onFuturama, orMilhouse had a talking Bender doll onThe Simpsons. This would seem to be another case when a popular franchise is acknowledged as fiction and not a crossover of the stories.

Perhaps the most obvious parodic crossover is found when characters from two series interact outsideeither series. This occurs most commonly on asketch comedy show or as a humorous interlude on an award telecast. Such crossovers may sometimes involve the real actors—for example, a sketch onRoyal Canadian Air Farce saw Yasir and Sarah fromLittle Mosque on the Prairie buying the gas station fromCorner Gas, with many of the characters in the sketch being portrayed by the shows' real actors—although they may also feature one genuine star from the show amid a cast comprised otherwise of the sketch show's own stable of actors.Parodic crossovers can be used to lendverisimilitude to the fictional world of a program. Characters from a fictional television series may appear on a stylized version of an established non-fictional television series, such asgame shows orreality shows. These crossovers between celebrity hosts and fictional characters are quite common onsituation comedies.Mama's Family once appeared onFamily Feud and the townsfolk ofThe Vicar of Dibley have had theirheirlooms valuated onAntiques Roadshow, for instance.

One of the earlier instances of crossovers in TV productions outside the US is the episodeUnter Brüdern [de] (1990), which was produced byWDR andDFF as a crossover between theWest German crime seriesTatort and theEast German crime seriesPolizeiruf 110. Their respective popular heroesHorst Schimanski and Peter Fuchs join forces to solve a case in the turmoil of the time after thefall of the Berlin Wall. The episode was produced during the short transition period between the fall of the Berlin Wall and thereunification of Germany.

• Thoughts On Crossovers In General (Kathryn Andersen) An essay musing on the numerous elements necessary to a successful crossover (primarily aimed at fan-written fiction).
• World Newton Universe Crossover Chronology Crossovers which build upon and take place in Philip José Farmer's World Newton continuity.
• "On Crossovers",Jess Nevins' history of the fictional crossover