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A shared universe is a fictional universe in which multiple works are set. One or more authors may contribute works to a shared universe. The works within the universe may share characters, settings, and other story elements, with or without continuity. Shared universes can be found in literature, comic books, films, television, anime and manga and are most commonly seen in fantasy, science fiction, comedy and a few more genres.
There is no formal definition of when the appearance of fictional characters in separate works constitutes a shared universe. Fiction in some media, such as most television programs and many comic book titles, is understood to require the contribution of multiple authors and does not by itself create a shared universe. Incidental appearances, such as that of d'Artagnan in Cyrano de Bergerac, may instead be considered literary cameo appearances. More substantial interaction between characters from different sources is often marketed as a crossover. While crossovers occur in a shared universe, not all crossovers are intended to merge their settings' back-stories and are instead used for marketing, parody, or to explore "what-if" scenarios.
The modern definition of copyright, especially under United States copyright law, considers the expansion of a previous work's setting or characters to be a derivative work. This often necessitates licensing agreements for new material being considered for publication (unless the author is the same). However, fair use claims have been raised and not all authors believe that fan fiction should be distinguished from other literature in this manner.
In a process similar to brand licensing, the intellectual property owners of established fictional settings at times allow others to author new material and create an expanded universe. Such franchises, generally based on television programs or film, allow for the creation and distribution of novels, video games, original sound recordings and other media based on the original product. However, not all shared universe settings are simply the expansion or combination of pre-existing material by new authors. At times, an author or group of authors engineer a setting specifically for development by multiple authors, often through collaboration.
Continuity methods and issues
It can become difficult for writers contributing to a shared universe to maintain consistency and avoid contradicting details in earlier works, especially when a shared universe grows to be very large. The version deemed "official" by the author or company controlling the setting is known as canon. Not all shared universes have a controlling entity capable of or interested in determining canonicity, and not all fans agree with these determinations when they occur. A fanon may instead find some degree of consensus within the setting's fandom.
Some writers, in an effort to ensure that a canon can be established and to keep details of the setting believable, employ tools to correct contradictions and errors that result from multiple contributors working over a long period of time. One such tool is retconning, short for "retroactive continuity", which resolves errors in continuity that came about through previously-written conflicting material. While most retcons serve to preserve continuity, a more severe form of retcon involves a wholesale rewrite of the groundwork for the entire setting.
Contributors to expanded universes, also known as tie-in writers, have sometimes been stereotyped as "hacks" because such writing is perceived as less creative or of consistently poor quality. These stereotypes have been disputed by authors who consider contributing to a larger work "intellectually demanding." The inclusion of professional and award-winning authors in this category have also done much to change public perception of writing in a shared universe.
Readers may also object when a story or series is integrated into a shared universe, feeling it "requir[es] one hero's fans to buy other heroes' titles", or leads to mischaracterizations and inappropriate comparisons.
Expansion of existing material
In 1941, writer Gardner Fox at All-American Comics (later part of DC Comics) created the Justice Society of America in All-Star Comics #3, credited with being the first superhero team-up and laying the groundwork for the DC Universe, the first comic book shared universe. By 1961, Marvel Comics writer and editor Stan Lee, working with artists Jack Kirby and Steve Ditko, merged the bulk of the publisher's comics characters into the Marvel Universe. Both settings have suffered from the creative difficulties of maintaining a complex shared universe handled by large numbers of writers and editors. DC has substantially altered its in-universe chronology several times, in series such as Crisis on Infinite Earths in 1985, Zero Hour in 1994, and Infinite Crisis in 2005. As of 2007, Marvel has rebooted its continuity only once, in Spider-Man: One More Day. They instead set stories in an increasing number of alternate realities, each with an assigned number in a greater "multiverse". DC and Marvel have also periodically co-published series in which their respective characters meet and interact. These intercompany crossovers have typically been written as self-limiting events that avoid implying that the DC Universe and Marvel Universe co-exist. Exceptions include the twenty-four comics released under the metafictional imprint Amalgam Comics in 1996, depicting a shared universe populated by hybridizations of the two companies' characters. Marvel has since referred to this as part of its setting's greater multiverse by labeling it Earth-692.
The Star Wars franchise takes a unique view regarding the canon properties of its expanded universe, introducing a four-tier system based on compatibility with the original six films. Star Trek canon is less well-defined, generally excluding not only licensed works such as books and video games and acknowledging that "even events in some of the movies have been called into question". Both franchises have blurred the lines between canon and non-canon content by adopting unofficial material into later official productions. 
The spin-off media extending of the universe originating in Doctor Who is particularly complex due to the permissive stance on licensing and canon taken by the BBC. This expanded universe has relatively little consistency given its division into audio plays produced by Big Finish and the BBC, the New Adventures universe novel, or a universe based on comics published in Doctor Who Magazine and other publications.
The expansion of existing material into a shared universe is not restricted to settings licensed from movies and television. For example, Larry Niven opened his Known Space setting to other writers initially because he considered his lack of military experience to prevent him from adequately describing the wars between mankind and the Kzinti. The degree to which he has made the setting available for other writers became a topic of controversy, when Elf Sternberg created an erotic short story set in Known Space following an author's note from Niven indicating that "[i]f you want more Known Space stories, you'll have to write them yourself". Niven has since clarified that his setting is still to be used only "under restricted circumstances and with permission", which Niven granted to the several authors of the Man-Kzin Wars series. By contrast, author Eric Flint has edited and published collaborations with fan fiction writers directly, expanding his 1632 series.
A setting may also be expanded in a similar manner after the death of its creator, although this posthumous expansion does not meet some strict definitions of a shared universe. One such example is August Derleth's development of the Cthulhu Mythos from the writings of H. P. Lovecraft, an approach whose result is considered by some to be "completely dissimilar" to Lovecraft's own works. Less controversial posthumous expansions include Ruth Plumly Thompson's and later authors' sequels to L. Frank Baum's Oz stories and the further development of Isaac Asimov's Foundation universe by Greg Bear, Gregory Benford, and David Brin.
Although DC and Marvel's successful shared universe approaches to comics have set them apart from competitors in the industry, other companies have attempted similar models. Valiant Comics and Crossgen both produced titles primarily set from their inception in a single, publisher-wide shared universe, known respectively as Unity and the Sigilverse.
Many other published works of this nature take the form of a series of short-story anthologies with occasional standalone novels. Examples include Robert Lynn Asprin's Thieves' World, C. J. Cherryh's Merovingen Nights and Janet Morris' Heroes in Hell.
Role-playing games are inherently designed to include some aspects of the shared universe concept, as individual games are derived from the core material. Campaign settings, such as Dungeons & Dragons's Faerûn, Dragonlance and Eberron, provide a more detailed world in which novels and other related media are additionally set. Living campaigns, including the RPGA's Living Greyhawk or the AEG-sanctioned Heroes of Rokugan, provide an opportunity for individual games hosted worldwide to take part in a single continuity.
The influence of the Internet on collaborative and interactive fiction has also resulted in a large number of amateur shared universe settings. Amateur authors have created shared universes by contributing to mailing lists, story archives and Usenet. One of the earliest of these settings, SFStory, saw its spin-off setting Superguy cited as illustrative of the potential of the Internet. Another example is the furry-themed Tales from the Blind Pig created at the Transformation Story Archive, which differs from many amateur settings both by having an organized effort to maintain consistent canon and by having seen some limited publication. Other early examples include the Dargon Project and Devilbunnies.
At least one publisher has introduced a division specifically for encouraging and handling shared universe fiction.
- The Universal Monsters cinematic universe (1931-1948; 2014-present) 
- X-Men and the Fantastic Four reboot 
- Transformers 
- Valiant Comics 
- Continuity and canon
- Expanded universe
- Fictional crossover
- Fictional universe
- Media franchise
- Setting (narrative)
- Spin-off and sister show
- "Tommy Westphall Universe" and "Wold Newton Universe" (hypotheses)
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