Massive Multiplayer Crossover



""So many heroes from so many dimensions! This is pretty cool!""

- Jenny Wakeman, Nicktoons: Attack of the Toybots

A Crossover that involves characters from more than two shows or more than two fictons.

More often than not, this is a mash up of series which do not have a strict sense of continuity or a clear Universe Bible. To lessen Canon-faulting, especially with series that do have strict continuity, a new 'neutral' setting is made that offers equal footing for all the characters.

This rarely occurs in live action shows, unless a production company can be formed that holds copyrights to everything. Thus, this is much more common in animated series -- although you can generally expect The BBC to pull one out of somewhere when Children in Need or Comic Relief rolls around.

It also becomes more viable the farther you get from Canon, such as one-time TV specials and especially video games (Kingdom Hearts, Jump Super Stars, Super Robot Wars, etc.)

As Story Arcs have become more prevalent, this practice has somewhat lessened, with shifts to strict Verse building and explicit references.

This trope has become increasingly common in video games, especially those involving both licensed and original properties. These games, depending on how far or how deep they mine, can have interesting effects on the fiction chosen. Many long-gone and/or forgotten Humongous Mecha shows, for example, often get a new lease on life, or even a brand-new sequel or remake, after making an appearance or two in a Super Robot Wars game. Similarly, the Fire Emblem series was finally brought over to the US to great success after two of its characters made an appearance as unlockable fighters in Super Smash Bros Melee.

In spite of its recently emerging prevalence, this trope is Older Than Feudalism. The Argonautica by Apollonius Rhodius (3rd century BCE) features nearly every ancient Greek mythical hero all going on a quest to find the Golden Fleece.

Sub Tropes:
 * Crisis Crossover (a company-wide Massive Multiplayer Crossover)
 * Cross Through (a company-wide event that affects every series involved without having them cross over)
 * Deconstruction Crossover (when the main purpose of using a Massive Multiplayer Crossover is Deconstruction)
 * Mega Crossover (the Fanfic version)
 * Monster Mash (classical movie monsters)
 * Ultimate Showdown of Ultimate Destiny (with few exceptions almost exclusively limited to Fan Fics)

See also Power Creep Power Seep and Story Breaker Team Up.

Anime and Manga

 * The Giant Robo OVA series featured characters taken from several other series Misuteru Yokoyama -- the original creator of Giant Robo/Johnny Sokko and His Flying Robot -- had written. This included adaptations of the medieval Chinese stories Romance Of The Three Kingdoms and The Water Margins, which led to many main and secondary characters in ancient Chinese clothing coexisting with people in three-piece suits Twenty Minutes Into the Future. It also included the very first Magical Girl, Mahotsukai Sally (Sally, the Witch), under her original name "Sunny", as Shockwave Alberto's daughter.
 * Legendary Manga creator Osamu Tezuka similarly uses Reused Character Designs, wherein a character from a previous work will actually play a different role in another story, as if they were an actor or actress. The Game Boy Advance game Astro Boy: Omega Factor pulls nearly all of them into one massive story.
 * DragonFall blended Dragonball, Star Wars and a whole bunch of other series into a parody. With mixed results.
 * The intro to the Rumiko Takahashi gallery show It's a Rumic World explicitly crosses over Ranma One Half, Urusei Yatsura, and Inuyasha.
 * Tsubasa Reservoir Chronicle has the characters jumping from Alternate Universe to Alternate Universe filled with CLAMP characters from their various series.
 * Amusingly, save a select few examples, mostly as cameos, the Alternate Universes are hardly ever populated by actual CLAMP characters, but Alternate Universe instances of them.
 * Their first crossover happened in Clamp Campus Detectives since the three main character were in series/stories on their own before CCD: Nokoru Imonoyama in Duklyon, Akira Ijuin in 20 Mask ni Onegai (Man of Twenty Masks) and supposedly Suou Takamura showed up in an old oneshot. Then there was X/1999 where characters from past CLAMP series started appearing, including Subaru, the main character of Tokyo Babylon, as a major character, and during his series set in the early 90s it was said that he would have a role in Endofthe World As We Know It that is X, making for a bit of Foreshadowing.
 * Kyoshiro to Towa no Sora is primarily a crossover of the mangaka and studio's previous works: Kannazuki no Miko, Steel Angel Kurumi, Magical Nyan Nyan Taruto, and UFO Princess Valkyrie.
 * Before Dengeki Gakuen RPG: Cross of Venus (mentioned below), there was this animated short produced for Dengeki Bunko's 2007 Movie Festival, featuring chibi versions of characters from Kino's Journey, Inukami and Shakugan no Shana (Note that the chibi Shana here is not Shana-tan; for one, she is the stalker rather than said omake series' Kazumi.).
 * While most Go Nagai series contain cameos here and there, the recent Shin Mazinger seems to be going the whole way. Characters from several alternate versions of Mazinger Z have shown up, as well as characters from Violence Jack (itself, a Deconstruction Crossover) and the titular demon from Mao Dante. Unsurprisingly, this series is being directed by the same man behind Giant Robo, Yasuhiro Imagawa.
 * Chibi Chara Go Nagai World was a crossover which featured SD versions of Mazinger Z, Devilman, and Violence Jack (with in-story explanations for the characters being SD).
 * The Pretty Cure All Stars movie series allows the Cures from the past seasons to meet (and be friends with) the new team.

Comic Books

 * The League of Extraordinary Gentlemen has a Superhero team composed of multiple public domain characters.
 * Crisis On Infinite Earths was a massive DC multiverse crossover that attempted to pare down the 837,000 alternate Earths (some populated by the superheroes DC Comics had acquired by buying out other comic book companies over the course of 50 years, others created just to resolve DC's own legendary Continuity Snarls) into one world, obliterating many "Alternate Earth" characters in the process.
 * Fables is about various figures from fairy tales and folklore living secretly in a neighborhood in NYC.
 * Planetary is about an investigative super-team in the Wild Storm universe. Their members have had run-ins with Dracula, Sherlock Holmes, Jenny Sparks from The Authority and multiple versions of Batman.
 * The Indelible Alison Bechdel offered a mash-up of various lesbian and gay comic artists, who threw their characters into the same world for a party. As the mash-up included Diane DiMassa, creator of Hothead Paisan: Homicidal Lesbian Terrorist, the results were hilarious.
 * Joe the Barbarian does this in a manner similar to The Indian In the Cupboard.
 * A non-canon Judge Dredd story in the 1980 Dan Dare annual had Tharg bring all the popular characters currently being published in Two Thousand AD as well as the Starlord to Dredd's apartment for a surprise party. Then the robots that actually write the comics went on strike, forcing the characters to beat them all up.
 * Superman and Batman versus Aliens and Predator
 * The Alan Moore comic Albion shoves together a whole bunch of British comic characters of varying obsurity, most of whom are imprisoned by the Government as part of The Masquerade.
 * Shi/Cyblade: The Battle for Independents featured many independent comic book characters, including Cerebus, Bone, Hellboy, Madman, Megaton Man, Scud the Disposable Assassin, The Tick and Usagi Yojimbo. Many of the same characters have also appeared in the Normalman/Megaton Man special, Gen 13 ABC and War Of The Independents.
 * Freddy vs. Jason vs. Ash, and its sequel Freddy vs. Jason vs. Ash: The Nightmare Warriors.
 * Dark Horse comics had Alien versus Predator versus The Terminator.
 * Fantastic Four: True Story has the FF traveling to the realm of fiction, that is attacked by Nightmare, lord of the dreamworld. To fight him they join forces with Dante Alighieri, the Dashwood sisters, Faust, almost all William Shakespeare's protagonists, Frankensteins Monster, Robin Hood, the Ivanhoe cast and others, while Nightmare gains allies in the Sheriff of Nottingham, Dracula and Long John Silver. Many other characters like Tarzan make cameos and in the end Johnny summons lawyer friendly versions of James Bond, Megatron, Steven Seagal and Chuck Norris to defeat Nightmare's army.

Commercials

 * A recent MasterCard commercial featured several food mascots (from Count Chocula to the Pillsbury Doughboy) eating dinner -- with Mr. Clean doing the dishes.
 * USA Network's commercials play this for laughs, having various combinations of characters from their shows (Burn Notice, Monk, Psych, others) encounter each other and make idle conversation.
 * UK example: The Greatest Minds In Advertising Join Forces in a 2009 viral for Comic Relief.

Film

 * Who Framed Roger Rabbit crossed over Disney characters with Warner Bros. and MGM characters, saying that they all co-existed with humans as actors in Hollywood, and that they all hail from "Toontown", which seemed to be half Los Angeles and half Alternate Universe.
 * The Monster Squad had Dracula as the Big Bad who controlled all of the other classic movie monsters: Frakenstein's Monster, the Wolf Man, the Creature From the Black Lagoon and the Mummy. There is also a cameo by Abraham Van Helsing.
 * The Area 52 scene in Looney Tunes Back in Action involved the heroes facing off against Marvin the Martian, who led a group of sci-fi aliens which included a Triffid plant, the eponymous Robot Monster, the mutant from This Island Earth and even freaking Daleks!
 * Many Seltzer and Friedberg works, such as Epic Movie and Disaster Movie, could be considered Massive Multiplayer Crossovers, insofar as they feature many characters and plot elements (or weak parodies thereof) from recent movies and mash them all together. By all rights, this really should produce something worth watching on some level.
 * A version of this in Star Trek Generations, which featured the captains of two Enterprises from completely different eras (Kirk and Picard) in the same film. Star Trek has done this numerous times, if you consider the different series separate entities of the same intellectual property.
 * Van Helsing, which features the eponymous monster hunter battling Dracula, a werewolf, Frankenstein's Monster, Igor and Mr. Hyde.
 * Done with toys in The Indian In the Cupboard.
 * And with horror-movie creatures in the Waxwork movies.
 * Toy Story features many toys from rival companies as major or minor characters including Mr. Potato Head, Barbie, and Legos among many others. (And probably some expies too.)
 * The Avengers serves as this for the Marvel Cinematic Universe.
 * In 1959 a movie was released called Alias Jessie James starring Bob Hope, who was a big fan of TV westerns; he used his clout to include characters from nine different shows in what would turn out to be the first of three Western massive multiplayer crossovers. People appeared in that movie from The Lone Ranger, The Gene Autry Show, Annie Oakley, Davy Crockett, The Life And Legend Of Wyatt Earp, Gunsmoke, Wagon Train, The Roy Rogers Show and Maverick.
 * In the 40th Anniversary OOO, Den-O, All Riders: Let's Go Kamen Rider has all of the main riders from Kamen Rider #1 to Kamen Rider OOO.

Literature

 * Science Fiction author Philip José Farmer's Wold-Newton universe includes scores of public domain characters as well as many characters popular from early Radio Drama and film, such as The Shadow and Tarzan, who are not quite out of copyright. Fans have added many modern TV characters to the list.
 * Philip José Farmer's Riverworld series does this with actual people from history.
 * Kim Newman once wrote a short story about Terry and Bob of the British sitcom The Likely Lads fighting in the Vietnam war with William of Richmal Crompton's old Just William stories and other fictional characters.
 * In fact, Kim Newman does this a lot, most notably Anno Dracula, which features just about everyone.
 * Silverlock by John Myers Myers, in which the protagonist A. Clarence Shandon is shipwrecked in the Commonwealth of Letters, where everywhere he goes and everyone he meets is a literary, mythical and/or historical reference.
 * A fairly extensive list of specific references can be found here.
 * Robert A. Heinlein's The Number of the Beast features a Time Travel device that does double duty as a portal into The Multiverse, allowing his characters to visit every fictional universe ever, including all of Heinlein's own novels. They coin the term "World as Myth" to describe the Recursive Canon necessary to make this work, and wind up hosting a convention for just about every Science Fiction character ever.
 * Spider Robinson's Callahans Crosstime Saloon series - including Callahan's Lady - contains cameos from characters created by crime writer Donald E. Westlake, SF legend Robert Heinlein, and even classic British humourist PG Wodehouse, all interacting with each other. (Most likely inspired by Heinlein's The Number of the Beast, mentioned above.)
 * Simon R Green's Nightside take place in a secret city under London that's a giant crossoverfest. John Taylor, Green's protagonist, has met characters from all manner of books, movies, television shows, and other assorted places though they are largely referred to in vague, shadowy terms so he doesn't violate the copyrights too badly. There's everything from a Traveling Doctor who had a trick with celery to having to exorcise Kandarian demons from his answering machine to giant 'bears of little brain' that work for the auction house.
 * For even more fun, representatives from most of Green's other series (the Deathstalker novels, the Darkwood books, etc) show up, waiting around to speak to Father Time.
 * While Neil Gaiman's short story A Study in Emerald is primarily a crossover between the works of HP Lovecraft and Arthur Conan Doyle, it contains subtle hints that characters such as Dracula, Frankenstein, and Dr. Jekyll also exist in the same universe.
 * A Night in The Lonesome October features Jack the Ripper, Sherlock Holmes, Dracula, Frankenstein, Rasputin, The Wolfman, and many others in a complex game determining whether or not Cthulhu and the other elder gods return to Earth. Interestingly, Jack is the hero...
 * And again in Roger Zelazny's book "Roadmarks" where Red runs across a short man with a small mustache whom Red refers to as "Adolph" driving a battered black Volkswagen, and later on in the book he makes a call to someone he calls "Doc", who is described as "A big golden-eyed guy with one hell of a suntan, wearing a torn shirt, and driving a hot little 1920's roadster" which could only have been Doc Savage.
 * Doc Savage villain John Sunlight also makes an appearance.
 * Zelazny was a comic book reader and fan. In _Blood of Amber_ Merlin has dinner at Bloody (Last Deceased Owner's Name)'s place--Boody Andy's at the time--while a gent (with a pronounced scar through his eye) eats at a neighboring table and warns Merlin to show a blade so the local roughs get no ideas. "Old John" was clearly John Ostrander and Tim Truman's mercenary John Gaunt (aka Grinner, Grim Jack) from Cynosure, a cross-dimensional city in a multiverse adjacent to Amber. The two roughs did not last the night.
 * The works of Dr. Seuss were combined into a Broadway musical called Seussical, which mainly takes its story from Horton Hears a Who, Horton Hatches the Egg, and The One Feather Tail of Miss Gertrude McFuzz, but contains elements and characters from I Had Trouble in Getting to Solla Sollew, The Butter Battle Book, How the Grinch Stole Christmas, and more. And of course, they have The Cat in The Hat to move the plot around.
 * Jim Henson's The Wubbulous World of Dr Seuss did something similar, with Yertle the Turtle as a recurring villain.
 * Stephen King's The Dark Tower series spans across the majority of his prior works.
 * The Harold Shea series of short stories are about Harold and company visiting various settings taken from mythology and public domain fiction, usually one per story.
 * Brazilian author Monteiro Lobato took this trope to insane levels in his kid's books set in the Yellow Woodpecker Farm. The eponymous farm is an interdimensional nexus to, essentially, every fantasy and adventure fiction character ever written, including but not limited to the Greek Gods, Sherlock Holmes, the Neverland people, the Arabian Nights, the fables from Aesop, Grimm, Andersen, The Three Musketeers, medieval Knighs etc etc etc ad infinitum. He even managed to throw in some characters copyright laws didn't allow him to. To top if off, characters native to the series' own universe are not few in number.
 * Jasper Fforde's like this trope a lot. Pretty much every Bookworld character in the Thursday Next series comes from another book. According to the rules of its universe every book crosses over with it. Including itself. Many of the characters from the Nursery Crimes series are right out of nursery rhymes.
 * Peter David wrote two novels where X-Men characters appeared in the Star Trek: The Next Generation universe.
 * L Frank Baum did this in the fourth book of the Oz series, The Road to Oz, by inviting characters from his other books to attend Princess Ozma's birthday party, hoping to get his Oz readers interested in those other non-Oz stories. This included everybody up to and including Santa Claus (as in The Life and Adventures of). The implication, of course, is that every book Baum ever wrote takes place in the same universe as the Oz books.
 * James A. Owen's Chronicles of the Imaginarium Geographica does this with pretty much every major work of fantasy, history, and real life. It's awesome.

Live Action TV

 * The "Night of Elizabeth Taylor", broadcast on CBS around 1995-96, saw a diamond necklace lost by Elizabeth Taylor became a common plot element linking four SitComs -- The Nanny, Cant Hurry Love, Murphy Brown and High Society -- in one massive crossover. It was intended as an embedded advertisement for Taylor's new perfume, Black Pearls.
 * Disney did a triple-episode MMC with three of its shows. The show was entitled That's So Suite Life Of Hannah Montana, with one classed as a Suite Life episode, one as a Raven episode, and one as a Hannah Montana episode, where Hannah and Raven visited the hotel the twins live in.
 * Likewise, there's the Wish Gone Amiss triple-episode, except it is more loosely tied together. It all involves the title characters from Cory in The House, The Suite Life of Zack and Cody, and Hannah Montana making a wish on apparently the same shooting star. Each episode has its own method of returning to the Status Quo -- Cory gets a literal Reset Button, Zack and Cody's wish was all just Zack's dream, and Hannah returns her life to normal when Jackson unwittingly wishes that the world did not know Hannah's double life.
 * There's also the Wizards On Deck with Hannah Montana that goes by the same formula that the That's So Suite Life of Hannah Montanna did, except with the stars visiting the hotel's ship instead of the hotel itself.
 * Kamen Rider Decade is this in regards to the Heisei era Kamen Rider shows. The main character dimension jumps into alternate universes based on the 9 previous Kamen Rider series of the last 10 years (as well as the canonical universe of Samurai Sentai Shinkenger). Plus, the first movie features every main Rider created before Decade, even ones that only had appeared in one-shot movies previously.
 * The second movie, a Grand Finale, even includes some Post Modernism commentary on the "interesting effects on the fiction chosen" mentionned in the opening paragraphs. To quote the original universe Wataru: "The tales of the Riders were something that would be eventually lost to time. But because of Decade's battles, they will remain fresh in people's minds..."
 * Similarly to Kamen Rider Decade, Kaizoku Sentai Gokaiger is a crossover series. But compared to Decade, in which the main character could only be Riders of the last decade, the Gokaigers can transform into any of the Sentai from the past 34 teams (barring Sixth Rangers - that is, until their own Sixth Ranger joined up). Additionally, while Decade established that every past Kamen Rider was its own universe, Gokaiger establishes that all Super Sentai took place in a single universe.
 * There's also Kaizoku Sentai Gokaiger vs. Space Sheriff Gavan the Movie - yeah, that means the Space Sheriff Trilogy is revealed to be set in the Sentai universe too.
 * Perfectly justified that Inazuma Ginga in Sun Vulcan episode 45 was wanted by the Galactic Police. Shozo Uehara who wrote the character would be the main writer of the Space Sheriff trilogy.
 * It all comes to a head with Kamen Rider X Super Sentai Super Hero Taisen.
 * The Children's Party at the Palace, a British production broadcast live from Buckingham Palace on June 25, 2006, celebrated Queen Elizabeth II's 80th birthday with a wild romp featuring dozens of literary and TV characters including (among many others) Cruella DeVille, Peter Pan and Captain Hook, Sir Topham Hatt from Thomas the Tank Engine, Wallace and Gromit, the White Rabbit from Alice in Wonderland, Enid Blyton's The Famous Five, Harry Potter, Ron Weasley, Hermione Granger and Neville Longbottom from the Harry Potter movies, and starred the Queen as herself.
 * The Sabrina the Teenage Witch episode "Inna-Gadda-Sabrina" extended its story through the following three shows that night: Boy Meets World, You Wish, and Teen Angel. Each show was set in a different time period in going with the theme.
 * It wasn't the first time - in fact, quite a few crossovers spanned the entire TGIF lineup from time to time, and even an occasional though less involving tie-in for all ABC weekly sitcoms during this period.
 * There was also one that was linked by Steve Urkel. He ends one episode of Family Matters by blasting through the Winslow's roof in a jet pack, and he crashes into the Lambert's roof at the beginning of the an episode of Step By Step where the plot centers around him. I forget if any other shows were connected as well.
 * A very simple one, but the iStart A Fan War episode of I Carly included characters from Drake and Josh and Zoey 101, helping to fill out the Nick Verse.
 * A Muppet Family Christmas included characters from all Muppet-related programmes: The Muppet Show, Sesame Street, Fraggle Rock and even Muppet Babies.
 * Its conclusion even featured a brief cameo of Jim Henson as himself.
 * In Doctor Who, the climax of the Second Doctor serial The Mind Robber. D'Artagnan vs. Cyrano de Bergerac! Blackbeard vs. Sir Lancelot! Plus Gulliver and Rapunzel on the sidelines.
 * Law and Order crossed over with Homicide Life On the Street a few times, until they eventually just decided they were set in the same continuity altogether, to the point of having John Munch, who originated in the latter show, permanently set up shop in the former.
 * The Tommy Westphall Hypothesis states that dozens if not hundreds of television series have all happened within the mind of a young autistic boy living in Baltimore.
 * John Munch is a central figure in this Hypothesis.
 * The Earth Day Special, which aired on ABC in 1990, was a huge crossover featuring just about every pop culture icon from The Eighties in a very bizarre, thoroughly nonsensical plot.
 * Also in 1990, Cartoon All Stars to The Rescue, the ultimate Very Special Episode combining well over a dozen Eighties cartoon characters. Aired once and only once, it's full of Narm yet also bizarrely entertaining. Rumor has it that it's never been aired since because Jim Davis claimed he hadn't authorized Garfield's inclusion in the show.
 * In the CSI-verse, the original series crossed over with the Miami series in 2002 to serve as the pilot for the latter; the Miami series then crossed over with the New York series in 2004 to serve as that series' pilot, and the series crossed over again in a two-hour storyline in 2005. The original series also crossed over with Without a Trace in 2007 a two-hour storyline across both series. Following the departure of William Petersen (who opposed the spinoffs and did not appear in any scenes featuring the Miami team in "Cross Jurisdictions"), CBS put together a massive crossover in 2009 spanning all three series that involved the Las Vegas series' Raymond Langston going to Miami and New York while investigating a human trafficking organisation.
 * The 2009 Two Pints of Lager and A Packet of Crisps Comic Relief special, which brought in characters from BBC Three's other sitcoms Grown Ups and Coming of Age. And yes, Sheridan Smith was Acting for Two.
 * The second western MMC took place a year after the first on what seemed to be an ordinary episode of Maverick called "Hadley's Hunters"; during the course of that hour he ran into people from 5 other shows: Lawman, Cheyenne, Bronco, Sugarfoot, and he stops by the office from Colt 45 but nobody was home (a reference to the show being recently canceled).
 * Strangely he also ran into the parking lot attendant from 77 Sunset Strip which was set in the 1960s -- I guess he had an ancestor who lived in the old west.
 * The final western MMC took place over 30 years later in the made for tv movie The Gambler Returns: The Luck of the Draw, where the title character crosses over with 10 different series. During the course of the film The Gambler meets people from Bat Masterson, The Life And Legend Of Wyatt Earp, Maverick, Cheyenne, Kung Fu, The Rifleman, The Westerner, The Virginian, and Rawhide, plus the main game was in honor of the main character from Have Gun Will Travel.

Mythology and Religion

 * The Argonautica (a.k.a. Jason and the Argonauts) by Apollonius of Rhodes (3rd century BCE) is one of the very first Massive Multiplayer Crossovers, arranged in what would become a fairly classic method -- basically throwing one or two dozen heroes from various separate Greek myth cycles together on a boat with a common mission. This of course makes the Massive Multiplayer Crossover Older Than Feudalism.
 * Many of the same characters also appear in the story of the Kalydonian Boar Hunt - which, Depending On the Author, may occur before or after The Argonautica.
 * According to some religious studies texts, this has also gone on in many, many other myths: the most notable involve various saints meeting each other. This goes on even today.

New Media

 * Weezer's Pork And Beans music video. It's got almost everything that ever appeared on Youtube short of OK Go and The Angry Video Game Nerd.
 * There's an updated version. It is as though the internet gave birth.
 * The Machinima "Beans". Firstly, it crosses over characters from three different series and is made by three different machinima directors, then the storyline involves various internet memes... Oh. And it's made on Super Smash Bros. Making it a crossover on a crossover.

Stand Up Comedy

 * There's a Star Trek-related comedy routine in which Mr. Spock, the HAL 9000 computer, and Obi-Wan Kenobi all appear on Jeopardy, in a mental variant of an Ultimate Showdown of Ultimate Destiny.
 * Stand-up comedy troupes sometimes feature a series of comedians who usually headline their own shows:
 * The Original Kings of Comedy
 * The Blue Collar Comedy Tour
 * The Comedians of Comedy

Tabletop Games

 * The Spelljammer and Planescape settings were designed with this in mind. Spelljammer in particular has rule books dedicated to detailing the Crystal Spheres of settings such as Krynn, Abeir-Toril, and Oerth. Where as Planescape has portals to every type of world imaginable.
 * Collectible Card Games that operate off of a Universal System allow for this; the Universal Fighting System lets you pit Chun-Li against Nightmare, the VS System crosses over Marvel and DC, and the Crusade System lets you pair Spike Spiegel, Edward Elric, and Cutie Honey with Humongous Mecha from Macross, Super Robot Wars Original Generation, and countless others.

Video Games

 * The King of Fighters was the original crossover fighter, taking characters from several SNK series including Fatal Fury, Art of Fighting, Psycho Soldier and Ikari Warriors.
 * Neo Geo Battle Coliseum is an extension of the above, including also characters from Samurai Shodown, Last Blade, World Heroes, Aggressors Of Dark Kombat and even Metal Slug!!!
 * This is effectively what defines the Mascot Fighter genre.
 * A staple of store demo displays in the later 90s, Sega produced Fighters Megamix, a crossover of Virtua Fighter, Fighting Vipers, and a couple one-off games such as Virtua Cop, Sonic the Fighters, and even Daytona USA.
 * Capcom's "VS" series:
 * The Marvel vs. Capcom games, (X-Men vs. Street Fighter, Marvel Super Heroes vs. Street Fighter, Marvel vs. Capcom Clash of the Superheroes, Marvel vs. Capcom 2 and now Marvel vs. Capcom 3) pit the characters of the various game series produced by Capcom against the heroes and villains of the Marvel Universe in, naturally, a fighting tournament. Only the sketchiest rationale is drawn for this.
 * Following this is the SN Kvs Capcom series, (SNK Vs Capcom The Match Of The Millennium, Capcom vs. SNK Millennium Fight 2000, Capcom vs. SNK 2 Mark of the Millennium and SNK vs. Capcom SVC Chaos) basically putting the characters of both companies' Fighting Game series against each other. A spinoff series known as SNK vs. Capcom: Card Fighters is a TCG pulling characters from both companies' entire libraries.
 * And then there is Namco X Capcom, a action/strategy RPG featuring characters from Namco and Capcom.
 * Street Fighter X Tekken and Tekken x Street Fighter however are actual fighting games featuring Namco (Tekken) and Capcom (Street Fighter) characters.
 * Only related through the VS. series via the appearance of one Capcom series rather than multiple but what the heck: Cross Edge is a RPG with Compile Heart at the helm of development. Characters from Darkstalkers, Ar Tonelico, Disgaea, Spectral Souls, Atelier Marie and ManaKhemia show up in this one.
 * There's also Tatsunokovs Capcom, which returns to the gameplay of the Marvel series but instead pits the Capcom characters against the works of Tatsunoko Productions like Gatchaman and Casshern. It is basically Marvel vs. Capcom using Japan's Marvel.
 * Project X Zone is an upcoming Spiritual Successor to Namco X Capcom, only now adding Sega (and technically Bandai of Namco Bandai, with Super Robot Wars Original Generation, and .hack being Bandai's franchises before the merger with Namco) into the mix.
 * Even before Tatsunoko vs. Capcom, there was a Fighting Game called Tatsunoko Fight, which pitted four Tatsunoko superhero anime (and one original made-up anime) characters, along with their females/sidekicks and villains.
 * Capcom's own Mascot Fighter: Capcom Fighting Evolution. The only game where we were able to have a Zangief vs. Alex fight, and which not only had the distinction of being the only Capcom-made crossover not featuring Morrigan, (Anakaris, Demitri, Felicia and Jedah, plus Pyron as one of the final bosses, were the Darkstalkers representants) but also the only game where we were able to fight as one (or two) of the Red Earth characters who aren't Tessa. (Leo, Hauzer, Hydron and Kenji were the representants)
 * In contrast, the Super Robot Wars series (almost certainly the originator of this trope in video game form) has magnificent, complex plots, and the individual games have touched upon nearly every Humongous Mecha series in existence. Particular of note is how they merge individual episodes to make missions, including having the famous "dancing" episode of Neon Genesis Evangelion set on Macross Island.
 * Additionally, Super Robot Wars Original Generation is a Massively Multiplayer Crossover of various Super Robot Wars original characters (as each Super Robot Wars sub-series is set in its own universe)
 * Furthermore, Super Robot Wars OG Saga: Endless Frontier is a crossover between Super Robot Wars Original Generation and Namco X Capcom (minus the Capcom) - making it a crossover between a crossover between crossovers and-- oh dear...
 * And to top it all off, Another Centurys Episode is Super Robot Wars using the system from Armored Core.
 * Chaos Wars uses a similar SRPG system to Super Robot Wars, but instead has more typical JRPG heroes from Atlus, RED, Idea Factory, and Aruze. This doesn't sound all that impressive -- outside of Atlus and maybe Idea Factory those companies are virtually unknown in the States, until you realize this includes Shadow Hearts (Aruze), Growlanser (Atlus), Spectral Souls (Idea Factory), and Gungrave (RED). A sequel has been talked about, and there are hints that Shin Megami Tensei/Persona characters may make it in, and that Nippon Ichi might be invited to join the other 4 designers in the next series.
 * And ditto Trinity Universe, which will have two story modes -- one with Nippon Ichi (Disgaea) characters, and one with Gust (Atelier Series) characters. It apparently revolves around someone throwing trash into Laharl's back yard. Gust and NIS are making a habit of doing these crossover games -- NIS also has a habit of localizing all of Gusts' releases in the states, too.
 * Yet another pastiche of SRW would be Battle Moon Wars which merges the three major published Nasuverse canons together: Tsukihime, Fate Stay Night, and Kara no Kyoukai.
 * Super Smash Bros has a number of first- and second-party Nintendo characters fighting each other - and the third iteration of the series, Super Smash Bros. Brawl, has taken it a step further, including Solid Snake from Konami's Metal Gear games and Mario's longtime rival, Sega's Sonic the Hedgehog.
 * There are also various fanmade Smash Bros. clones, then most notable example being the Super Smash Flash series for its inclusion of more third-party characters and anime characters.
 * Square-Enix's Kingdom Hearts series merges nearly every movie in the Disney Animated Canon and some outside of it with the feel and characters of the Final Fantasy series (though, just like the Tsubasa example above, the Disney characters are alternate universe incarnations, as implicitly stated in the Tron world).
 * On the PSP is Dissidia Final Fantasy, where heroes and villains of various Final Fantasy games face off as part of the machinations of two opposing powers.
 * Tales of the World, Namco's set of Massive Multiplayer Crossover games for their Tales Series. One bonus, though, is that the Narikiri Dungeon heroes' ability to dress as any Tales character and gain their abilities applies to bosses, too. Now add in the fact that theme music changes to whatever character or costume is in the lead of a party in battle, and you have a winning idea when a Tales fan can battle to Motoi Sakuraba's more popular themes, like Fighting of the Spirit or Decisive.
 * A recent download introduces Taki and Mitsurugi from Soulcalibur for use as well.
 * Tales of VS has recently been announced. It's a Fighting Game that we have been told is going to have a total of 35 characters from 13 different Tales games. So far only 8 have been announced. They are: Stahn Aileron and Mighty Kongman from Tales of Destiny, Kratos Aurion from Tales of Symphonia, Richter Abend from Tales of Symphonia Dawn of the New World, Luke fon Fabre and Anise Tatlin from Tales of the Abyss, Shing Meteoryte from Tales of Hearts, and Yuri Lowell from Tales of Vesperia.
 * Tales of Hearts itself is a borderline example. The cameo summons are somewhere between the trope and a very prominent Shout Out.
 * As noted above, Astro Boy: Omega Factor for the GBA contains characters from many of Osamu Tezuka's works. You gain levels by meeting new characters.
 * Cartoon Network: Fusion Fall is a Cartoon Network MMORPG that combines their various original series (including Codename Kids Next Door, Ed, Edd n Eddy, The Powerpuff Girls, and Ben 10, among others).
 * A.C.E.: Another Century's Episode is, much like Super Robot Wars, a massive crossover between a number of Humongous Mecha series. It's an action game rather than a turn-based strategy game, though.
 * Mortal Kombat vs. DC Universe, where blood-thirsty kombatants and superheroes beat up each other due to a Hate Plague spread by an ugly fusion of two Big Bads.
 * THQ's Nicktoons Unite series, merging various Nicktoons with Spongebob SquarePants, Danny Phantom and The Adventures of Jimmy Neutron consistenly featuring as well as reoccuring appearances of varying frequency from The Fairly Oddparents, Tak and The Power of Juju and Invader Zim (And that's long after the last one has been killed off, mind you). Oh, and I kid you not, Stimpy and Rocko in the console version of Nicktoons: Attack of the Toybots, and Aang in Nicktoons Nitro. it has been sorta-followed by Nicktoons MLB.
 * Mobile Suit Gundam: Gundam vs. Gundam is an arcade Fighting Game with a Crisis Crossover Excuse Plot that throws all thirty years of the Gundam franchise together so characters like Amuro, Domon, Heero, Kira, and Setsuna can beat the stuffing out of each other.
 * Similarly, there is  that has several stories focusing on different pilots teaming up into two forces because of this. Eventually, you can even have different pilots using different mecha.
 * Dengeki Gakuen RPG: Cross of Venus is a Nintendo DS Action RPG that throws Shakugan no Shana, Iriya no Sora UFO no Natsu, Kino's Journey, To Aru Majutsu no Index, Nogizaka Haruka no Himitsu, Bludgeoning Angel Dokurochan, Toradora and Asura Cryin' (with cameos from Spice and Wolf, Baccano!, Inukami! and the Boogiepop Series) together into a Crisis Crossover.
 * Dream Mix TV World Fighters is a fairly bizarre crossover Fighting Game featuring characters from Konami, Hudson Soft, and Takara. It's a shame it wasn't very good; where else could you pit Bomberman, Simon Belmont, Solid Snake and Optimus Prime against each other?
 * Magical Battle Arena is a 3D flying Fighting Game that pits the female mages and Magical Girls of Magical Girl Lyrical Nanoha, Cardcaptor Sakura, Slayers, and Mahoujin Guru Guru against one another, with more series likely to be included in future Expansion Packs.
 * Jump Super Stars has characters from dozens of Weekly Shonen Jump series in a Super Smash Bros-style battle royale.
 * Similarly, Battle Stadium D.O.N. is the same, but specifically has characters from Dragonball, One Piece, and Naruto (hence the D.O.N.).
 * Speaking of which, Shounen Sunday and Shounen Magazine are looking to take a bite out of Jump Super Stars together with Sunday VS Magazine Shuuketsu Choujou Daikessen (Also known as Sunday vs. Magazine) on the PSP.
 * Quake III (and by extension, Quake Live) included the Doom guy, the Quake marine, the Quake II marine and the guy from Wolfenstein 3D (named Sarge here) among other characters as playable characters. Of course, they were all of the Silent Protagonist variety to begin with, so this didn't make a huge difference.
 * The mod Generations Arena takes the above concept and runs with it, extending the crossover to the gameplay via five classes, each adapting the gameplay of said five games to the Quake III engine, right down to the finest details. Dooms fast-paced battles and BFG against Quakes flexible air control and powerful rocket launcher? You got it.
 * And that's not even including custom skins, with which the crossovers skyrocket through the roof.
 * MUGEN is a customizable 2D fighting engine that permits end users to homebrew reproductions of any other character in the entire genre, and create unique ones. Fighters already exist for every "vs" title released and almost every fighting games, and can be crossed over in unique ways.
 * Nippon Ichi has recently released a new one in Japan with a release coming in summer 2010 to the US, Trinity Universe. It's made in conjunction with Gust and Idea Factory.
 * Also, Disgaea 3 has characters from almost all Nippon Ichi Series. Including the entire cast of the earlier Disgaea games. Makai Kingdom, Soul Nomad, Phantom Brave and La Pucelle
 * This Game Mod for Civilization IV has an All Fiction is True premise, allowing for a player to fight wars and build empires as several famous fictional leaders ranging from Agamemnon to Macbeth to Rama, Dracula, Rufus T. Firefly, and Jed Bartlet. Wonders of the World include Galt's Gulch, Hogwarts, Camelot, and Jurassic Park, and adds Hero Units like Don Quixote, Sun Wukong and The Time Traveller.
 * The Square Enix board game series Itadaki Street has character crossovers of Dragon Quest with either Final Fantasy or Super Mario depending on the game. Though traditionally No Export for You, a Western release has been announced for the Wii as Fortune Street.
 * Mentioned by the Nerd himself (well, his actor), Super Mario Bros Crossover.
 * The fan-made Mushroom Kingdom Fusion combines a wide variety of classic and contemporary video game characters and universes together into one game. For starters, playable characters include Mario and Luigi, Wario, Sonic and Tails, Link, Arthur, and Roll.
 * Indie Brawl, a Fighting Game à la Super Smash Bros, but with characters from the Indie Gaming culture.
 * Poker Night At the Inventory, a Poker game involving characters from Penny Arcade, Sam and Max Freelance Police, Team Fortress 2 and Homestar Runner.
 * In theory, expansions or further installments could potentially include characters from just about any video game you could name (in practice, any character owned by a company too chicken to go in on a project solely on the basis that it's a cool idea can be struck from the list).
 * Any Wii game that uses Miis as playable characters or NPCs can become this if you make enough of them.
 * Playstation Move Heroes, featuring Jak and Daxter, Ratchet and Clank, and Sly Cooper (and Bentley).
 * Play Station All Stars Battle Royale takes this concept, bolts on a whole lot more franchises, and storms Super Smash Bros.'s territory with a Mascot Fighter.
 * GwangeDash, an indie shmup featuring ships, enemies and bosses from a lot of different shooters, including DoDonPachi, Guwange, Ketsui, Ikaruga, Twin Cobra, Gradius, Battle Garegga, Radiant Silvergun and countless more.
 * The Macross Frontier PSP game series allows you to play all Macross universes. The recent title, Macross Triangle Frontier, covers Macross Zero, Macross (as well as  Do You Remember Love), Macross Plus, Macross 7, Macross Dynamite 7, Macross Frontier (as well as Macross Frontier: The False Songstress) and Macross 2.
 * In a more literal example, Minecraft allows anyone to make their own character skins, which leads to 20+ characters all working together to build a house/civilisation and mine for diamonds.
 * Cartoon Network Punch Time Explosion is a Smash Bros. clone with Cartoon Network characters. (BTW, it's not a clone just because it's a crossover either. It literally has the same gameplay, right down to tilt/smash attacks, Final Smashes, assist characters, and the percantage point system.)
 * Heroes Phantasia is an upcoming RPG from Banpresto, featuring Keroro Gunsou, Slayers, Darker Than Black, Mai Hi ME, Sorcerer Stabber Orphen, Read or Die, Rune Soldier Louie, S-Cry-ed, and Blood +.
 * Konami's MSX2 Mahjong game Hai no Majutsushi featured eight mascots from various other Konami games: Popolon, Aphrodite, Pentarou, Goemon, Dr. Venom, a Snatcher, a moai, and a perverted version of Simon Belmont.
 * NES pirate cartridge Kart Fighter drops the characters from Super Mario Kart into a fighting game, while World Heroes 2 (not to be confused with the actual World Heroes games) includes Ryu, Chun Li, M. Bison, Haggar, Andy Bogard, Lawrence Blood, Mai Shiranui, Mario, King Koopa, Sonic the Hedgehog, Leonardo, and Goku.
 * The Warriors Orochi games has the characters of both Dynasty Warriors and Samurai Warriors dropped into the same setting due to the Big Bad messing around with time to seek out powerful warriors to challenge him. Along for the ride are some mythical beings exclusive to the series and by the third game, Ayane, Ryu Hayabusa, Achilles of Warriors Legends of Troy, Nemea of Trinity Souls of Zill Oll and Jeanne d'Arc of Bladestorm.

Theater

 * Seussical has characters from several of Dr. Seuss' books

Webcomics

 * There have been at least two webcomic Massive Multiplayer Crossovers: The Framed!!! Great Escape and the recently ended Crossover Wars
 * Though it never crossed over with both at the same time, Its Walky! had crossovers with both Melonpool and Fans! that established both as part of its universe (or multiverse in the latter case) and which ended up having a lasting impact on its storyline.
 * Kevin and Kell and General Protection Fault had an arc where two GPF characters wandered into K&K's Domain world. The artists managed to work the events into both their storylines that were running at the time, and apparently drew a strip each day--both had two-strip days where the storyline advanced.
 * Starslip Crisis: Alterverse War is a mashup of a large number of science fiction webcomics, and had just started as of October 2007.
 * The webcomics Queen of Wands and Something Positive had a couple of crossover stories. Later, after QOW ended its run, Kestrel showed up as a semi-regular on S*P.
 * Choo-Choo Bear hired the Pet Professional from the webcomic of the same name to kill his homicidal cousin Twitchy-Hug.
 * And recently Davan from S*P has been seen texting with Candy from Girls with Slingshots after running into her at the wedding of Jameson and Maureen.
 * Choo-Choo Bear later had kittens with Sprinkles of Girls with Slingshots, one of whom Davan gave to Roz of Shortpacked!, and another is by Hazel to her cousin Robyn from All New Issues.
 * Even better, none of those strips have been shy about crossovers of their own. QOW drags in Punch and Pie, which stars its Breakout Character Angela; It's Walky pulls in Fans and Avalon.
 * Questionable Content has crossed over with Diesel Sweeties, which has itself crossed over with Scary Go Round. Questionable Content has also crossed over with Applegeeks, which, having crossed over with Ctrl Alt Del, would turn the whole group into one quasi-incestuous ball of webcomics. I'm reasonably certain that Pv P has been referenced in at least one of these, probably Diesel Sweeties, and since the former has crossed over with Penny Arcade more than once, it's completely cross-eyed insane over there.
 * And the Author of Ctrl Alt Del exists as a person in the Penny Arcade universe, so that means, if you follow the little red thread of universal strings, that he exists in the same plane of existence as his characters..
 * Don't forget that Applegeeks had a major crossover (well, on its end) with Megatokyo at one point. Which chains the whole thing to a specific date, making things even crazier.
 * It gets better. Questionable Content is now having background extras as characters from other comics like Girls With Slingshots, Octopus Pie, Anders Loves Maria and Overcompensating. Since Overcompensating features the author and most people known in the webcomic industry, it means all these characters now exist in the same universe as their creators.
 * The cast of Shortpacked has visited the Coffee of Doom coffee house from QC on occasion, including once when both artists did a take on the same scene. Combine that with the S*P Intercontinuity Crossover Nexus mentioned above, along with all their respective crossovers, and we have a massive webcomic universe that could give Marvel a run for its money.
 * Sluggy Freelance hosted one of these during the Filler Arc "Sluggy Freelance: Where Are You?" When all of Sluggy's actors are kidnapped (a.k.a. Pete Abrams decides to take some time off) characters from several other webcomics are hired to fill in for them and/or find the kidnappers.
 * Several members of the Webcomics Inc. social networking site collaborated on a crossover comic, where they made all their characters teen-aged and put them into high school together. The result, WCI High, merged characters from Good Times, Mikey's Life, Everyday Heroes, and Big Sandy Gilmore.
 * Least I Could Do recently finished the Ultimate Final Civil War Invasion Crisis Thing, which featured the gaming webcomic characters trying to kill the slice-of-life comic characters. (Order of the Stick, meet Xkcd.)
 * Ménage à 3 and Sore Thumbs crossed over at some point and are implied to be in the same universe, and Ménage à 3 crossed over with School Bites. If this keeps up we'll wind up with every webcomic crossing over to one another.
 * The writer of No Rest For The Wicked footnotes her comics so you can get all the fairy tales she's using characters from.
 * Final Blasphemy has characters from Mega Man, Final Fantasy, Mario, and several other gaming universes.
 * Troops of Doom has G.I. Joe/Cobra and Star Wars Empire characters, as well as other random characters and an alien race called Legonians.

Web Original
": (speaking to the various reviewers present) In fact, I think there's a lot of you who want to do crossovers, aren't there? Because everybody really eats that shit up.
 * Although there are many similarly-themed RPs, Milliways Bar deserves mention just for its size. The basic premise is that any character from any fandom can find themselves at the titular bar... and sometimes it seems like just about everyone has.
 * The Game in which Life in A Game takes place seems to take all video games as true, with Zelda appearing alongside Master Chief, and Frog serving as the hero's Jerkass Mentor.
 * The That Guy With the Glasses one year anniversary video. Basically, in a Chicago hotel 20 contributors of Video Review Shows beat up one another in an Ultimate Showdown of Ultimate Destiny, climaxing in blowing up. You know what? Just watch it.

Everybody: Yeah!"


 * All the cast stayed in Chicago for a couple more days to film crossovers after this. Highlights include Linkara being force-read his own Massive Multiplayer Crossover fanfiction by the Sage, the Ultimate Warrior writes a comic series so bad that reality breaks down and Linkara and Spoony keep changing into different Alternate Universe selves and the Critic and Nerd joining forces to review... a Making Of of a Teenage Mutant Ninja Turtles Tour. Yeah.
 * Later on when faced with Uwe Boll's Alone in The Dark, the Nostalgia Critic gave up on reviewing it. Until! Spoony and Linkara arrived to assist him.
 * And kicking off 2010, we have an epic review of Dragon Ball Evolution, starring Paw, LordKat, Rollo T, Hopewithinchaos, and Y Ruler of Time, with guests including LittleKuriboh, MasakoX and The Happy Video Game Nerd. This was recorded while everyone was attending MAGFest.
 * The Two Year Anniversary is another huge crossover event. And in turn, it allowed some once-in-a-lifetime crossovers as by this point, quite a few reviewers are from around the world.
 * In 2011, things repeated. Y Ruler of Time used MAGFest to do another multiple reviewer crossover (The Last Airbender, with Todd in The Shadows, Jesu Otaku and Rollo T, plus various cameos). And Year Three had a crossover series and various individual joint reviews (Todd even opens his one saying that the trip means crossovers... and gets shot down by three people before Film Brain accepts to watch a movie with him).
 * How about The Ultimate Showdown of Ultimate Destiny flash video? Good guys; bad guys; explosions as far as the eye can see. Way too many pop culture icons to count.
 * This roleplay, taking place right here on our own forums. Princess Tutu, Naruto, Pokémon, The Legend of Zelda, No More Heroes, and Discworld- and that's just for starters.
 * It has its own page now. Although technically I'd say it's more of a Mega Crossover.
 * Symphony Of The Blood, a fake video game crossover between all the characters of the Star System of Osamu Tezuka.

Western Animation

 * Disney owns all the characters it uses plus a whole network. The most recent example is House of Mouse, which has Mickey and Co. as hosts of a nightclub/theater, with the characters of the feature films as the audience.
 * Many classic Hanna-Barbera characters have been used across the board, in shows like Laff-a-Lympics and Yogis Gang; this is still done, but with a more satirical bent (e.g., Harvey Birdman Attorney At Law).
 * Nearly all recent Warner Bros animated television series made references and overt cameos with one another.
 * As stated above, the early-90s public service announcement Cartoon All Stars to The Rescue had characters like the Teenage Mutant Ninja Turtles, The Smurfs, Winnie the Pooh, the cast of Duck Tales, and many others team up to teach a single kid about the dangers of drugs . Although, if you can see Smurfs, there's a good chance you're tripping balls already.
 * The '80s cartoon Defenders of the Earth starred King Features' most famous adventure-hero characters: Flash Gordon, The Phantom, and Mandrake the Magician (along with his sidekick, the strongman Lothar).
 * Much earlier, in 1972, the one-off animated special The Man Who Hated Laughter united all of King Features' popular characters -- meaning not only Flash, the Phantom, Mandrake, and Lothar, but also the likes of Popeye, Blondie, Snuffy Smith, and Beetle Bailey.
 * Captain N the Game Master featured a mishmash of characters who appeared on the Nintendo Entertainment System done wrong. Most of the episodes took place in the neutral Videoland, with characters like Simon Belmont, Mega Man and Kid Icarus Pit hanging around, even if In Name Only. Link and Zelda from The Legend of Zelda (more accurately, from the Zelda cartoon) appeared too.
 * The Children in Need music video "Peter Kay's Animated All-Star Band", which features nearly every significant British Stop Motion characters with a few 2D British and American characters broadcast "live via satellite", including Roary the Racing Car, Fifi and The Flowertots, The Wombles, In The Night Garden, Angelina Ballerina, Scooby-Doo, Bagpuss, Bob the Builder, Ben 10, Thunderbirds, Postman Pat, SpongeBob SquarePants, Fireman Sam, Camberwick Green, and Paddington amongst others.
 * All singing parts of the characters by their original voice actors. The video took two years to produce, and it's for charity.
 * Done with advertising mascots in the short film Logorama. Better Than It Sounds; it won the 2009 Oscar for Best Animated Short.
 * The Rosey And Buddy Show, a 1992 TV special produced by Nelvana, featured not only animated versions of Roseanne and Tom Arnold, but also the Care Bears, Beetlejuice, Tom&Jerry, Droopy and even Wile E Coyote and The Road Runner's stunt double.