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A fictional crossover (short: crossover) is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders, unauthorized efforts by fans, or common corporate ownership.
Official crossovers 
Crossovers often occur in an official capacity in order for the intellectual 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.
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 the public domain with those concurrently under copyright protection.
A crossover story may try to explain its own reason for the crossover, such as characters being neighbors (notable examples being the casts from Golden Girls and Empty Nest) or meeting via dimensional rift or similar phenomenon (a common explanation for science 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' respective continuities. Still others intentionally make the relations between two or more fictional universes confusing, as with The Simpsons and Futurama, where each show is fiction in the other.
Crossovers of multiple characters owned by one company or published by one publisher, have been used to set an established continuity, where characters can frequently meet within one setting. This is especially true of comic book publishers, as different characters in various Marvel, DC or Valiant comic 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, Batman and Superman often collaborate. In comic book terminology, these "guest star" roles are common enough that they are not considered crossovers. A crossover in comic book terms only occurs when a story spans more than one title. This has led to "crossover events", in which major occurrences are shown as affecting most or all of the stories in the shared universe.
The earliest such crossover event was Gardner Fox's Zatanna's Search, which took place in Hawkman #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), and Justice League of America #51 (February 1967). This story dealt with Zatanna attempting to reconnect with her father, Zatara, and seeking the aid of Hawkman, Batman, Robin, The Atom, Green Lantern, and Elongated 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 the Secret Wars crossover which brought all the major Marvel heroes into a twelve issue mini-series to battle a common threat. After the threat was dealt with they all returned to their regular titles. This 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 of Spider-Man's black suit which would later become the villain Venom). Jim Shooter later perfected his crossover at Valiant Comics with the Unity event. Unity brought all the Valiant characters together to defeat Mothergod 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. Like Secret Wars, the Unity crossover had lasting effects on the Valiant universe most notably the introduction of Turok, the birth of Magnus Robot Fighter and the death of a major Valiant hero.
Dark Horse Comics's Aliens Versus Predator franchise was a success that continued into many games and two movies and even an Aliens Versus Predator Versus The Terminator comic.
The comic crossovers from Raj Comics are very famous in India, in which the super heroes meet to fight a common enemy. Many of these crossovers have occurred between Nagraj and Super Commando Dhruva. In Kohram, all the heroes in Raj Universe meet to finish Haru, an extremely powerful enemy.
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 is Cartoon Network's The Grim Adventures of the Kids Next Door. It features 5 crossovers - Ed, Edd n Eddy, Codename: Kids Next Door, The Grim Adventures of Billy and Mandy, a reference to The Powerpuff Girls and a quote from Scooby-Doo, which are all licensed Cartoon Network series. The cast of Ben 10: Ultimate Alien and Generator Rex team up in Ben 10/Generator Rex: Heroes United. Most of the last episodes of the Lilo & Stitch: The Series (a spinoff of the eponymous Lilo & Stitch) had crossovers with various other Disney cartoons, including The Proud Family, Kim Possible, Recess and American Dragon Jake Long, although one might count them as mere cameos, as each plotline took place exclusively in Lilo's hometown in Hawaii. The Jimmy Timmy Power Hour trilogy is another example, in which Jimmy Neutron and Timmy Turner switch universes. Another crossover is Rugrats Go Wild in which the Rugrats are stranded on an island where The Wild Thornberrys were at the time.
During the 1970s and 1980s, crossovers were particularly common among the Hanna-Barbera properties. Some of the earliest examples happened on The New Scooby-Doo Movies, which featured appearances by characters from Harlem Globetrotters, Josie and the Pussycats, Jeannie, Speed Buggy, Batman and Robin, and The Addams Family. Later, the Hanna-Barbera Superstars 10 set of movies involved several crossovers, including such combinations as The Jetsons Meet the Flintstones. This was taken to an extreme in the 1977-79 series Laff-A-Lympics, which was essentially a gathering of the most popular H-B characters (at that time) 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 famous movie Who Framed Roger Rabbit had characters from various companies, most notably Disney and Warner Bros. Daffy Duck and Donald Duck made a simultaneous appearance in one scene, in which the two of them exchanged blows during a piano duet. Later in the movie, Mickey Mouse and Bugs Bunny were shown parachuting together (to keep things from getting too iffy legally, Mickey and Bugs' lines were written so that each of them said exactly the same number of words in the movie). The film also includes cameos of characters from MGM.The end of the movie features all the cartoons from all of the animation companies joining together in song, to be concluded by Porky Pig stuttering his famous "That's All, Folks!" line as Tinker Bell ends the scene with a magical fade-out.
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, and NBC aired this half-hour special one Saturday morning with characters from all their networks, including Huey, Dewey, and Louie (from Disney's DuckTales), Winnie the Pooh, Tigger, Slimer (from The Real Ghostbusters and Extreme Ghostbusters), Michelangelo (from Teenage Mutant Ninja Turtles), Bugs Bunny, Daffy Duck, Alvin and the Chipmunks, The Smurfs, ALF (from his short-lived cartoon spinoff), Garfield, and the trio of Baby Kermit, Baby Piggy, and Baby Gonzo (from Jim Henson's Muppet Babies). Animation companies granted unlimited, royalty-free use of their cartoon characters for this project, a feat that has been unequalled before or since then. This cartoon was also introduced by then-President George H. W. Bush and Barbara Bush, and would be distributed to schools and video stores free of charge nationwide.
Manga artist Leiji Matsumoto has been known to crossoveer the characters of his various stories and characters such as Captain Harlock, Galaxy Express 999, and Queen Millenia, all of which were originally written as separate, self-contained stories. In the Maetel Legend, Queen Promethium is revealed to be having been the Yukino Yaoi, protagonist from Queen Millenia. Matsumoto has also created various crossovers with Space Battleship Yamato, an anime on which he served as director, although the rights to Yamato is actually owned by Yoshinobu Nishizaki.
The adult parody-oriented series Drawn Together features many crossovers; while some are regular cameos, such as Peter and Lois Griffin from Family Guy, others involve a personality twist, such as the homicidal version of Bambi from the eponymous film or the gay version of Elmer Fudd from the Looney Tunes franchise.
The upcoming Disney/Marvel crossover Phineas and Ferb: Mission Marvel features the Marvel heroes Spider-Man, Iron Man, the Hulk, and Thor, the Marvel villans Whiplash, MODOK, Red Skull, and Venom, and characters from Phineas and Ferb, where the storyline involves Phineas and Ferb trying to restore power to the Marvel heroes, whose powers were taken away by Doofenshmirtz.
Video games 
Konami made the first crossover video game featuring Simon Belmont from Castlevania & Mikey from the Warner Bros. Movie The Goonies in Konami Wai Wai World for the Famicom in 1988. The King of Fighters, Marvel vs. Capcom, and many other franchises from third-party developers such as Capcom and SNK bring these licenses together.
The 2.5D fighting game series Super Smash Bros., brings various Nintendo characters together for a massive fight. The third game in the series, Super Smash Bros. Brawl introduces the first non-Nintendo characters with the inclusion of Solid Snake and Sonic the Hedgehog, of Konami's Metal Gear series and Sega's Sonic the Hedgehog series respectively.
The Namco-developed Soul series, features characters from outside of Namco's titles (Link from Nintendo's The Legend of Zelda series, Necrid, Heihachi Mishima from Tekken, Todd McFarlane's Spawn, Star Wars's Darth Vader, Yoda, and Starkiller, Lloyd Irving (from Tales of Symphonia), and Ubisoft Montreal's Ezio Auditore (from the Assassin's Creed series)). Mario & Sonic at the Olympic Games, released in Japan two months before Super Smash Bros. Brawl, was the first time that Mario and Sonic (as well as their associated characters) appeared in a game together. In the RPG Kingdom Hearts, Disney and Final Fantasy characters join forces.
Super Robot Wars are turn-based strategy games featuring a variety of Japanese mecha series from many generations, such as Mazinger Z, Gundam, Neon Genesis Evangelion, and GaoGaiGar. More recently, there has been a collaboration between Capcom and Namco, which will result in the upcoming Street Fighter X Tekken and Tekken X Street Fighter. The Disney film Wreck-It Ralph features many video game character cameo appearances.
One of the first film crossovers was Frankenstein Meets the Wolf Man, in 1943. This was also perhaps the beginning of a film legacy of monsters brawling each other, such as House of Frankenstein and its sequel, House of Dracula. In 1948, the comedy film Abbott and Costello Meet Frankenstein was released. This led to Abbott and Costello Meet the Invisible Man, Abbott and Costello Meet Dr. Jekyll and Mr. Hyde, and Abbott and Costello Meet the Mummy. Then, Toho released King Kong vs. Godzilla in 1962. Forty-one years later in 2003, Freddy vs. Jason was released by New Line Cinema. Freddy vs Jason sparked speculation of a Chucky vs Leprechaun film, as well as the rumored Helloween, which would combine Hellraiser and Halloween.
In literature, some authors also engage in crossovers by including characters from different novels they have written in one particular volume; L. Frank Baum did this regularly, and Michael Moorcock frequently uses this device - particularly in his Eternal Champion series of novels, which establish a vast 'multiverse' populated by numerous different characters, many of whom appear in different novels and even different genres. Kim Newman is another author who frequently uses this device, as does Stephen King. The works of James Branch Cabell, J.D. Salinger, William Faulkner, Margaret Laurence, Kurt Vonnegut, Mordecai Richler, Edgar Rice Burroughs and Isaac Asimov also 'crossover' with each other, linking different characters and settings together over a number of different works.
Brazilian writer Monteiro Lobato also created solid and imaginative crossovers, using elements and characters from Brazilian folklore such as the Cuca and Saci, from Greek Mythology, from the Arabian Nights, from Fairy Tales such as Grimm's Snow White, western literature such as Peter Pan, cartoons such as Popeye and Felix the Cat, and western films.
Public domain 
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; a prominent example of this occurs in Loren D. Estleman's novel Sherlock Holmes vs. Dracula, in which Sherlock Holmes and Dracula are brought together and pitted against each other. The League of Extraordinary Gentlemen by Alan Moore and Kevin 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 of Philip José Farmer's Wold 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 between Sherlock Holmes and Tarzan). Roger Zelazny's novel A 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 within the first volume of The League of Extraordinary Gentlemen, 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 creator Sax Rohmer, he is not directly named as such in the work and is only referred to as 'the Devil Doctor'.
Television series 
Crossovers between established shows 
||This section may contain excessive, poor, or irrelevant examples. (February 2012)|
Crossovers involving principals can also occur when the characters have no prior relationship, but are related by time period, locale or profession. For example, the Law and Order series afford a commonality of setting which lends itself to crossovers, and they are frequent within the Law and Order franchise.
Following the cancellation of the ABC soap opera One Life to Live and its high rated finale, several characters crossed over into the network's remaining soap opera General Hospital, remaining in the same timeline as their former show.
Shows with the same production companies, distributors and/or television networks 
Though most common on shows of the same production company, crossovers have also occurred because shows share the same distributor or television network. A notable example of this kind of link is that between Murder, She Wrote and Magnum, P.I. These shows were made by different companies, but owned by Universal Studios and broadcast on CBS. Another case is that of Mad About You and Friends, which share the character of Ursula 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 followed Mad About You on NBC's Thursday night schedule).
Mad About You and Friends 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 City blackout caused by Paul Reiser's character on Mad About You was experienced by the characters on Friends. 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 its Friday night programming during the 1997 season. There, they proposed that the title character of Sabrina the Teenage Witch should chase her cat, Salem, through Boy Meets World, You Wish and Teen Angel because it had run away with a "time ball" that was displacing each show through time.
Narrative rationales for crossovers 
On other occasions, crossovers between established shows can occur without a network or production commonality, but simply because there is some narrative rationale for the crossover. The appearance of detective John Munch (from NBC's Homicide: Life on the Street) on Fox's The X-Files happened merely because the episode revolved around a crime scene in Baltimore, a logical place for characters on The X-Files to have encountered Munch. Munch would also appear on the TV series Law & Order on NBC in which it had one episode which began on that series in New York City and concluded in Baltimore on Homicide: Life on the Street. Later, when Homicide went off the air in 1999, Detective Munch ends up leaving Baltimore to move to New York, and becoming a permanent character (as New York City Detective Munch) on NBC's Law & Order: Special Victims Unit.
A two-part crossover episode between CSI: Crime Scene Investigation with Without a Trace aired on November 8, 2007. The first hour was on CSI and the second hour was on Without a Trace. While both series are on the same network in the United States, spreading two parts of a story across two different shows can cause problems in international markets where they have been separately sold to different broadcasters. For example, in the United Kingdom, Without a Trace was shown on Channel 4 and CSI on Channel 5, meaning the two companies had to come to a special agreement to show both episodes. Another CSI crossover occurred in 2009 when Raymond Langston from CSI: Crime Scene Investigation appeared both in CSI: Miami and CSI: NY, traveling to Miami and New York to track a human trafficking and organ harvesting ring. ER had a crossover with Third Watch which corresponded with a peak in viewership for the latter show, with 17.2 million viewers.
Crossovers in children's television 
Disney Channel's Suite Life franchise has featured three crossovers: in 2007, "That's So Suite Life of Hannah Montana" featured characters from Hannah Montana and That's So Raven taking a vacation at the Tipton Hotel (the main setting of The Suite Life of Zack & Cody); Hannah Montana/Miley Stewart (Miley Cyrus) also eats cake off Zack, which recurs in the 2009 crossover episode "Wizards on Deck with Hannah Montana", in which characters from Wizards of Waverly Place and Hannah Montana come on aboard the S.S. Tipton. Characters from The Suite Life on Deck also appear in a crossover with fellow Disney sitcom I'm in the Band (an unusual crossover given the two series air on different networks: The Suite Life on Disney Channel, and I'm in the Band on Disney XD) in the 2010 episode "Weasels on Deck." "Wizards On Deck with Hannah Montana", which premiered on July 17, 2009 on Disney Channel, was seen by more than 9.3 million viewers, becoming the number one program of the night across both cable and broadcast television, and ranks as one of the highest-rated episodes for a Disney Channel original series.
On December 7 2012 the special Austin and JESSIE and Ally All Star New Year featured characters from the show Austin and Ally and Jessie in one big event as an episode for both shows. Trish (Raini Rodriguez) books Austin (Ross Lynch) to play in Times Square on New Year's Eve. Austin, Ally (Laura Marano) Trish and Dez (Calum Worthy) get sent to the wrong city and couldn't pay enough for the far distance. Meanwhile Emma (Peyton List) convinces Jessie (Debbie Ryan) to go see Austin in person. They bump into each other, Jessie gets Austin to stage and they invite them to their Penthouse. Jessie uses Zuri's (Skai Jackson) poem to use as a hit for Austin and Zuri gives her credit and Austin and Jessie perform a Duet together and Dez post's it on the internet.
Another type of crossover occurred in a 2010 episode of the Nickelodeon sitcom iCarly; the episode "iStart a Fan War" featured recurring characters from two Nickelodeon series: Drake & Josh characters Eric Blonowitz, Craig Ramirez and Gavin Mitchell and Zoey 101 character Stacey Dillsen. This was possible because all three series were created by the same producer, Dan Schneider.
Special usages 
Promotional cameos 
Crossovers can take the form of a promotional cameo 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 is The Simpsons episode A Star Is Burns, in which the character of Jay Sherman (from The Critic) appeared. This episode was largely condemned by fans of The Simpsons as existing to promote The Critic. Even Simpsons creator Matt Groening objected, preferring to remove his name from the credits of that particular episode in protest.
In 2010, as a nod to the 50th anniversary of Coronation Street, characters in fellow established TV soap EastEnders made reference to watching the special anniversary episode. EastEnders had celebrated its own 25th anniversary earlier in the same year.
In its simplest and most common form, a television crossover involves a starring character on a parent show appearing on a spin-off or vice versa because of established character relationships. An obvious example of this type of crossover occurred when Cliff Huxtable of The Cosby Show visited his daughter, Denise, on A Different World. Another example of this is the appearances made by Buffy the Vampire Slayer characters to Angel in Los Angeles from Sunnydale.
Depending on the complexity of the franchise, though, the crossover can be even more indirect, serving to suggest the size of the fictional world. This crossover is common in the Star 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 the Klingons Kor (from the Original Series episode "Errand of Mercy"), Koloth (from "The Trouble With Tribbles") and Kang (from "Day of the Dove"). After the passage of about a century of narrative time, the three onetime adversaries of Captain Kirk appeared together in the Star 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. Another Klingon, Arne Darvin, appeared as a secondary character in "The Trouble with Tribbles", but was the principal villain of DS9's "Trials and Tribble-ations". Intended as a celebration of the franchise's 30th anniversary, "Trials and Tribble-ations" was actually a crossover in and of itself; using then brand-new bluescreen techniques, the episode places the DS9 cast (after hijacking the Defiant, Darvin had gone back in time hoping to kill Kirk and become a hero to the Klingons, and it's up to Sisko & Co. to stop him and preserve the timeline) inside the TOS episode, interacting with Kirk, Spock and the rest of the Enterprise crew.
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 whether Out of the Blue is a spin-off of Happy Days, or whether the star of Out of the Blue merely crossed over into Happy Days.
Parodic crossovers 
Often, the problems of bringing together two shows with different narrative ambitions make the writing of a crossover burdensome. Such difficulties are encountered by situation comedies that wish to crossover with dramatic television programmes. The satirical crossover—ranging in length from a cameo to a full comedy sketch or episode—is an extremely popular way of circumventing this problem. By various means, such crossovers typically avoid outcry from fans by being obvious parody or homage. However, on rare occasion, the humor of such crossovers can be used by one show make a narrative point by capitalizing on the audience's experience of the other programme.
Such tongue-in-cheek crossovers typically fall into one of several broad categories.
- Parodic crossovers can be directly established as being outside of the continuity of one or all of the properties being crossed over. A good example is the crossover between The Simpsons and The X-Files, which was largely accepted as being outside of standard X-Files continuity.
- One episode of The Grim Adventures of Billy and Mandy shows that after Mandy smiles-Mandy, Billy and Grim are transformed into The Powerpuff Girls with a cameo by Professor Utonium.
- They can occur by virtue of a dream 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 of Newhart had been the dream of Bob Newhart's character on The Bob Newhart Show. It has more recently been used to demonstrate that cast members of The 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. Characters from King of the Hill have appeared on The Simpsons to comment on a peewee football game. Gag cameos may also include the appearance of an actor from another show, but not necessarily the character that the actor played. For instance, on the ABC/CBS show Family Matters during the closing credits of the episode "Scenes From a Mall" (Season 5, Episode 12), a scene which was shown earlier in the episode featuring Reginald VelJohnson is re-played, but this time with one of the child actors stating that he "looks like that fat guy from Fresh Prince," referring to James Avery who played Judge Phillip Banks on NBC's The Fresh Prince of Bel-Air. To the obvious surprise of the studio audience and VelJohnson, Avery walked onto the set with an angry look, being in on the staged joke himself. Ended the episode (but with the cameras filming still), VelJohnson and Avery hugged and smilingly greeted the public.
- Crossovers of this type can also be completely wordless. This type of crossover is more common on animated programmes, such as when Bender found and ate Bart Simpson's shorts on Futurama, or Milhouse had a talking Bender doll on The 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 outside of either series. This occurs most commonly on a sketch comedy show or as a humorous interlude on an award telecast. Such crossovers may sometimes involve the real actors — for example, a sketch on Royal Canadian Air Farce saw Yasir and Sarah from Little Mosque on the Prairie buying the gas station from Corner Gas, with all 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. Such crossovers are generally immediately apparent as parodies to the audience — and in no way considered a part of either show's continuity — due to the need for the hosting show to approximate the sets and costumes of the satirized programmes quickly and inexpensively. When Patrick Stewart appeared in a Star Trek: The Next Generation/The Love Boat crossover on Saturday Night Live, for instance, few Star Trek fans would have been fooled by the visual design into believing the event "counted" as an episode of the show. However, there are some cases of this type of parody having some canonical resonance with viewers. For instance, the British charity appeal, Comic Relief often contains parodic crossovers of a technically higher quality than the typical sketch show. Many of these Relief sketches are produced by the cast and crew of the actual programmes being parodied, and hence appear to be "normal" episodes. A good example of this is the sketch, "BallyKissDibley", an 11-minute piece in which the leads of Ballykissangel appeared on the sets of The Vicar of Dibley, alongside most of Dibley's cast. Since the sketch derived its humor from all actors remaining in character, the extent to which these parodies "count" as part of either show's canon is more open to interpretation than most sketch crossovers.
- Parodic crossovers can be used to lend verisimilitude to the fictional world of a programme. Characters from a fictional television series may appear on a stylized version of an established non-fictional television series, such as game shows or reality shows. These crossovers between celebrity hosts and fictional characters are quite common on situation comedies. Mama's Family once appeared on Family Feud and the townsfolk of The Vicar of Dibley have had their heirlooms valuated on Antiques Roadshow, for instance. In such cases, it is generally the non-fictional show which ends up being the most satirized, due to a need to compress the experience to its most recognizable elements. However, these crossovers can happen on dramatic television, such as when Blue Peter provided narrative exposition on The Sarah Jane Adventures. Rarely, brief crossovers between two fictional programmes can be used for this same purpose. In the episode, Army of Ghosts, Peggy Mitchell was seen in a fictionalized scene from EastEnders in order to demonstrate the degree to which the titular ghosts had permeated the popular culture of Doctor Who's Britain. Here, too, time constraints caused the satire of the guest programme (EastEnders) and not the host programme (Doctor Who).
Retroactive crossovers 
Sometimes, crossovers occur even when there was no explicit intent to create them. Viewer interpretation can play into the size and complexity of crossovers. These sorts of crossovers involve no creation of additional material, but merely result from inferences drawn about existing filmed episodes. Usually they are a product of narrative ambiguity. Perhaps the best example of this was caused by the unusual ending to St. Elsewhere. One interpretation of the ending scene of the final episode has been that the entire run of the program was a figment of autistic character, Tommy Westphall's imagination. This leads itself to a broad interpretation of the events of that series. Because the show had direct crossovers with twelve different programmes, and each one of these twelve had numerous other crossovers, linkages can be found from Elsewhere to 280 other shows, comprising what has been called "the Tommyverse".
Unofficial crossovers 
In contrast with legal crossovers, unofficial crossovers are created solely because of the artistic pleasure derived by its producers. Generally, unofficial crossovers take the form of fan-written fiction and fan art, but it is increasingly prevalent in amateur films and audio. 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 to enjoin the distribution of such material.
Unofficial crossovers can also occur in a "what-if" scenario. The most notable is an episode of Family Guy entitled "Lois Kills Stewie" which turned out to be a simulation along with the previous episode "Stewie Kills Lois". Stewie Griffin is confronted by Stan Smith and Avery Bullock from American Dad! while hacking into the central power grid at the CIA in a latest plan for world domination. Stewie mistakes Stan for Joe Swanson, due to notable similarities between the two shows. Stewie threatens to turn off all the electricity in the world until Stan and Bullock fulfill his demands of being "President of the World", which they do. Even though this never really happened, this is considered by some to mark the first official appearance of American Dad! characters in a Family Guy episode. Roger makes frequent cameo appearances in Family Guy, while Brian makes cameos on American Dad!. Roger, Rallo Tubbs, and Klaus Heissler were seen in the final Family Guy Star Wars spoof, "It's A Trap!", as an Imperial Officer, Nien Nunb, and Admiral Ackbar, respectively. Stewie also appears as an interactive hallucination of Booth on Bones 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.
Sherlock Holmes's War of the Worlds introduces the famous detective to the scene of London occupied by Martian invaders, as depicted by H. G. Wells, the crossover facilitated by the fact that both works, set in late Victorian London, are now in the public domain and can be freely used and modified.
Fan fiction crossovers between different science fiction movies and series are often created, such as Star Wars vs. Star Trek or Babylon 5 vs. Stargate. 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.
See also 
- List of crossovers in fiction
- Amalgam Comics
- canon (fiction)
- Callback (comedy)
- Continuity (fiction)
- fictional character
- fictional universe
- Intercompany crossover
- shared universe
- Tommy Westphall
- Wold Newton family
- Menefee, Craig (March 19, 1998). "Comic Strips Crossover On The Net". New Straits Times. p. 38. "lasted two weeks"
- epguides.com listing for Pandora's Box, the episode which kicked off "Blackout Thursday" on NBC.
- Friday night programming with Sabrina the Teenage Witch and her cat, Salem.
- "A CSI Without a Trace Crossover". CSIfanatic.com. Retrieved 2007-07-31.
- "Bone Voyage". CSI: Miami. Season 8. Episode 7. 11-09-09.
- "Hammer Down". CSI: NY. Season 6. Episode 7. 11-11-09.
- Various reactions, including Matt Groening's, to A Star Is Burns
- 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