The use of "canon" to describe the degree to which a work adheres to the standards of its fictional world, appears to have originated amongst devotees of the Sherlock Holmes stories, as a way to distinguish between the original works of Arthur Conan Doyle and adaptations of those works or original works by other writers utilising related characters and settings. However, much of the interest in and controversy over issues of canonicity have appeared in recent decades in the fan followings of films and television shows, such as science fiction franchises Star Wars and Star Trek.
When the body of work nominally set in the same fictional universe becomes large enough, it can happen that new material, such as might be found in spin-off television shows, prequels and books, contradicts earlier material. Such contradictions may be a result of bad research, or an attempt to revise, correct or retcon a perceived error in earlier material. The question is which material to favour and which to ignore when attempting to resolve all the material into a consistent whole. Two simple approaches are the "principle of first mention" in which information in the original work provides a foundation which later material must respect, and the revisionist model in which the latest work always supersedes earlier material. However, the situation can be much more complicated.
Generally, "Expanded Universes" are not considered canonical; by analogy with the idea of a canon of Scripture (see Biblical canon), such stories are considered "apocryphal". However, there are exceptions. In the case of the Star Wars canon, the Expanded Universe is canonical, though open to interpretation in a way which the "gospel" of the films is not. Doctor Who, which began life as a television series but has also been produced in prose, audio and graphical formats, has never had a single author or authority to pronounce on the issue of canon, and its fans run a spectrum between those who consider only some parts of the television series canonical and those who consider everything labelled as Doctor Who canonical.
In addition, a story can belong to two overlapping canons. One of the most obvious examples of this is Philip José Farmer's Wold Newton family. Some (but not all) of the adventures of Sherlock Holmes, Tarzan, Doc Savage, etc. are canonical in the Wold Newton setting. This does not mean that the events of Farmer's books are canonical from a Sherlockian perspective. Similarly, fans of Laurie R. King's novels of Sherlock Holmes and Mary Russell consider all the Holmes stories to be canonical in King's setting.
The difference can be even less clear-cut than this. Current Star Trek novels maintain a tight continuity with each other, and avoid contradicting the television series. When a Lost Era novel set between the movies and The Next Generation features a younger version of a character introduced in a Deep Space Nine novel, it's obvious there's some sort of "canonical" novel-setting, even if the TV series is not obliged to conform to it. This is where fanon and canon often collide, especially when a TV series, movie or other officially canonical source contradicts it. An example is the Trek novel Starfleet: Year One, which appeared in print before the TV series Star Trek: Enterprise was announced, but was completely invalidated by the series. Generally, though, in the case of televised fiction, only facts which appear in the as-originally-aired version of a program are considered canonical (including scenes cut from re-runs, but not including such things as deleted scenes and scenes from unaired pilots and other such material that 'leaks out' over the Internet).
Furthermore, the issue is also complicated when the definition of a canon changes well after the fictional universe is established. As an example, in the 1970s and 1980s, there were a number of reference works published by Franz Joseph and FASA Corporation for Star Trek. These books were considered canonical at the time (some even made with the explicit approval of Gene Roddenberry), sanctioned by Paramount Pictures, and were used almost universally by novel and comic book authors, as well as the production staff of the earlier Star Trek movies (information from these manuals appeared as background dialogue in some scenes, and many diagrams were used as computer displays). However, in 1988, as part of the release of Star Trek: The Next Generation, Gene Roddenberry and Paramount Pictures changed their policies regarding canonicity and stripped these books of their canonical status, as the new series quickly made many changes and revelations which openly contradicted earlier canonical books. Thus, a book that would be considered completely canonical in 1985 like The Star Trek Star Fleet Technical Manual, would be considered non-canonical in 1995.
In some fictional universes, interviews and other communications from authors are also considered canonical — like the letters of J. R. R. Tolkien with relation to Middle-earth; also items such as interviews, Internet chat sessions, and websites (e.g., the website of J. K. Rowling in relation to the Harry Potter series). This usually only happens in cases where all works in the universe have the same author.
In almost all cases, fan fiction is not considered canonical, as fan fiction is usually produced by amateurs. Sometimes, however, events or characterizations portrayed in fan fiction can become so influential that they are respected in fiction written by many different authors, and may be mistaken for canonical facts by fans. This is referred to as "fanon". The use of fan fiction to fill gaps or continuity errors in a canon is derisively called "fanwanking," or "fanwank." (The terms "fanon" and "fanwank" can apply to officially-licensed works, as well.) An intentional inversion of the exclusion of fan fiction came in Eric Flint's 1632 universe; in February 2000, fans and other established authors were invited on the Internet forum Baen's Bar to shape the multiverse, and the fan-fic, once vetted, is itself published in the various Grantville Gazettes, themselves under the direct editorial control of Flint and a 1632 editorial board. This is an ongoing process that apparently will continue indefinitely, as the series continues to burgeon in popularity.
Additionally, works of foreign origins (as is the case with most Japanese-produced video games, manga or anime) may have certain details of the original plot changed or modified during the adaptation from one language to another. The person in charge of the adaptation may choose to write an adaptation canon in addition to the original canon to maintain consistency when adapting a possible later work such as a sequel or a spinoff, although this is not always the case. An adapted version of the same work can sometime deviate completely from its source material, resulting a separate franchise from the original, as is the case with the Macross and Robotech franchises.
Unique to the Babylon 5 universe among virtually all other shared universes is the sanctioned canonicity of many of its offshoot novels and comic book stories; nearly all of the Babylon 5 novels and novelizations to date having been based on outlines written directly by J. Michael Straczynski. The later Del Rey books are considered to be more canonical than some of the earlier Dell ones, although – per Straczynski's own remarks – canonical elements exist in every single book published to date; Straczynski's deeper involvement in the novel-publishing program from 1996 onward having ensured a greater level of canonicity within such works. This is particularly significant the Passing of the Technomages trilogy which addresses plot points and contains revelations that were originally intended to be included in the cancelled Crusade TV series. Other books, including To Dream in the City of Shadows and the Centauri Prime series pick up significant plot threads that were beyond the focus of the series. It is a rare early case of multi-media story telling. In order to know the full story in addition to watching the main series and its televised spin-offs it is necessary to read many of the novels and comics.
The Buffyverse canon consists of the television series Buffy the Vampire Slayer and Angel, as well as the comics The Origin, Fray, Tales of the Vampires, Tales of the Slayers, Buffy the Vampire Slayer Season Eight, and Angel: After The Fall. All of the tie-in novels and video games, along with most of the comics, are largely considered either non-canon or apocrypha. Some of the comics are written by members of Mutant Enemy writing staff; the canonical status of these materials is still unclear. The 1992 movie is not considered canon. A comic adaptation based on Whedon's original script for the film was released, entitled The Origin, and this is considered canon instead.
Several spin-off titles, of both individual titles in the main series as of the main series itself, have also been produced. They are designated as such because they are stated by their developers as taking place in their own distinct alternate timelines and/or universes, which do not directly connect or contribute to the progressing story portrayed within the main series.
Most, but not all, comic books published by DC Comics (and some titles of the Vertigo Comics imprint) take place in a shared world known as the DC Universe. The canon of this world comprises all the post-Crisis comics not stated to be set in an alternate universe, except those specifically contradicted by later stories following Zero Hour (most notably, Batman: Year Two, Batman: Son of the Demon and the Action Comics Weekly strip featuring Captain Marvel). The events may not have occurred exactly as shown, however, owing to the floating timeline.
Appearances of the DC Comics characters in other media are not considered canon; for example, the appearance of a Marvel Comics character, Jigsaw, during a Marvel/DC comics publishing crossover, is apparently not considered a piece of canon for the adventures of Jean-Paul Valley, aka Azrael, who at one point took up the role of The Batman. Jigsaw was an enemy of Frank Castle, a Marvel Comics character called The Punisher.
Following the events of the second Crisis (2005) and 52 (2006/7), Batman: Son of the Demon has been written back into continuity as evidenced by Batman & Son. Also many Elseworlds stories such as Kingdom Come and Batman: Gotham by Gaslight are now canon, albeit in self-contained alternate universes. All the stories of the WildStorm Universe also exist in this new Multiverse. Stories set in the animated universe adapted from DC Comics stories were also integrated into an alternate reality.
The concept of canon plays an important role in the fantasy role-playing game Dungeons & Dragons (D&D). Over the years, a number of campaign settings have been published for D&D, each detailing a world or worlds that provide a setting in which the game can be played. There are two types of canon issues that arise from this situation.
The first issue is intra-setting canon, which deals with the backstory, locations and gameplay dynamics considered to be canon within a specific setting. Because D&D generally falls into the fantasy sub-genre of sword-and-sorcery, canon discrepancies can arise beyond just the typical issues in fiction of history, plot and character. For example, debates regarding canon within a given setting may include such arcana as the source of various magical powers and how they operate within the setting. Theological issues related to the various deities that may exist within the setting occur, as do disagreements regarding the canon cosmology of the setting's universe.
The second type of canon problem that arises in D&D is inter-setting canon, concerned primarily with the relationships between different settings. Many D&D publications have discussed how the various settings are related to each other within the D&D multiverse of various planes of existence, although these relationships are debated or even denied by some fans of the game.
There is no single "official" canon for D&D. From the very beginnings of the game in the 1970s through to the present, the issue of canon has been left up to each individual Dungeon Master, who runs the game session for the other players. The Dungeon Master is free to determine which published materials (adventure modules, novels, sourcebooks, video games, comic books, etc.) are canonical in his or her own campaign, and how the various D&D rules apply to that campaign.
Nevertheless, D&D players often move between games managed by different Dungeon Masters, and many also congregate for gaming tournaments, play in shared living campaigns, or play the game online with different participants than in their normal gaming sessions. In order to achieve even a basic level of continuity among these various game instances, D&D fans must therefore confront the issue of canon. To achieve the desired level of continuity, various mechanisms are employed to manage D&D "canon." The organizers of gaming tournaments, for example, will often specify which sets of rules and conventions are "in force" for tournament gameplay. Living campaigns may attempt to develop a more comprehensive set of canon materials and sourcebooks, such as the Living Greyhawk Gazetteer for the Living Greyhawk campaign. And for individually-run games, it is common for Dungeon Masters to briefly discuss their own vision of D&D canon with each new player who joins his or her campaign.
Finally, following the emergence of the World Wide Web, a number of websites have arisen that enable players to discuss canon issues and work toward (or reject) canonical norms. These include the website of Wizards of the Coast, the intellectual property rights holder and publisher for D&D, as well as fan-run sites such as EN World and Canonfire!.
The most expansive of these universes is the original Universal Century timeline, which features numerous side stories which take place alongside the main series. For example, the OVAs Gundam 0080: War in the Pocket and Gundam: The 08th MS Team are stories set within the same time period of Mobile Suit Gundam's One Year War but have nothing to do with the characters or situations in Mobile Suit Gundam. Similarly in the Anno Domini timeline, the manga series Gundam 00F is a side story to the anime series Mobile Suit Gundam 00 in much the same way.
Alongside the seven canonical timelines, there exists several other non-canonical timelines, such as the universes featured in Super Deformed Gundam and Superior Defender Gundam Force. There also exists a number of other media set within the seven official timelines but not considered part of the canon of those universes; these non-canonical media usually come in the form of various manga, novels or video games rather than anime. For example, the novelization of Mobile Suit Gundam changes many aspects of the original anime and is thus not part of the Universal Century canon, despite the novel being written by Yoshiyuki Tomino, the original creator of the Gundam series.
Since the conclusion of the Harry Potter series, much attention has also been paid to Rowling's comments on things never directly mentioned in the books, such as the homosexuality of major character Albus Dumbledore. Rowling's statements in interviews and web chats on what `happens' to the books' characters beyond the scope of the novels is also accorded canon status, despite that she has made clear that she does not intend to officiate this information by adding further books to the series.
Alternate universes in Marvel Comics include, for example, the "Ultimate" line of Marvel comics, which have their own canon independent of the core Marvel universe.
Appearances of the Marvel Comics characters in other media are not considered canon. One of the few exceptions is the video game version of Ultimate Spider-Man, which was made with the intention of being canon.
Defining the Middle-earth canon is difficult, because many of J. R. R. Tolkien's key writings were not published in his lifetime. A considerable number of Tolkien fans do not believe that a canon can be defined at all, preferring to observe the evolution of Tolkien's stories in the many versions and drafts published posthumously in The History of Middle-earth series. Most, however, agree that The Hobbit and The Lord of the Rings are canon, and also include a substantial amount of material published in The Silmarillion, Unfinished Tales, and other posthumous books, as well as information from Tolkien's letters. Works outside of canon include fan fiction (like Nick Perumov's "Ring of Darkness"), art books (except for the collections of Tolkien's own art) and video games; the Lord of the Rings movies by Ralph Bakshi and Peter Jackson are generally considered non-canonical, as well. However, there are a group of Tolkien fans, sometimes called "Continuationists" who consider Roleplaying games, Video Games and the films as canon, unless they contradict with Tolkien's own works.
Defining the Land of Oz canon is done in many different ways. The term "canon" for the Oz books was used as early as 1976, in The Oz Scrapbook by David L. Greene and Dick Martin. James Thurber considered only the first two books canon, while some consider only the first book canon and the sequels to be moneymaking exercises. The series was continued by other writers who openly took credit for their work, unlike, for example, the Stratemeyer Syndicate, which has caused many a fan, especially a young fan, to aspire to write their own additions to the canon. While many consider the fourteen L. Frank Baum books to be the true canon, most often, including for Greene and Martin, the canon is referred to as the forty books published by Reilly & Lee, including the fourteen books of Baum (though Reilly & Lee did not publish The Wonderful Wizard of Oz until it fell into the public domain in 1956), the nineteen by Ruth Plumly Thompson, three by longtime illustrator John R. Neill, two by Jack Snow, one by Rachel R. Cosgrove and one by Eloise Jarvis McGraw and Lauren McGraw Wagner. Some include even more books, particularly the additional books by these authors, including one by Baum (another is hard to reconcile with the canon, however), two by Thompson, two by McGraw, one by Cosgrove, and one by illustrator Dick Martin. Some would go further and add the books of Eric Shanower, who illustrated some of these "deuterocanonical" books (as The International Wizard of Oz Club refers to them), particularly since his writing and illustrations are generally both well-received. This is still a matter of much debate and one there is no firm agreement on. For example, The Life and Adventures of Santa Claus is very important to the world-building of the countries that surround Oz and the immortal races that live on the continent, but some have excluded it from canon because Santa Claus is tied to Christianity and others because it is not in accord with what later Oz writers, including Thompson and Robin Hess, have written about Santa Claus. The Oz books of Gregory Maguire are highly contradictory of their sources, and should be considered heretical to the canon, and not retcons. The Wizard of Oz 1939 film is far too contradictory to the novels, including the first, to be canon, though it is not unusual for apocryphal books (notably Wright, Tedrow, and Roger S. Baum), and Maguire to a certain extent, to use it as a source.
The Sherlock Holmes canon consists of the stories and novels written by Arthur Conan Doyle. This was decided by the Baker Street Irregulars, a group of Holmes enthusiasts, to distinguish the original stories from the pastiches that followed Holmes' retirement, and is probably the first use of the word in this context. However, certain Conan Doyle items were disregarded for other reasons and additions to the current canon of sixty mysteries have been discussed.
The Star Trek canon consists of the television series Star Trek, Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager, Enterprise, and the eleven Star Trek movies. Originally, there was little official policy on canon, and Star Trek: The Animated Series and some books like The Star Trek Star Fleet Technical Manual were apparently canonical (and excerpts from them were even used onscreen in the early movies). However, circa 1987, when Star Trek: The Next Generation was debuting, Gene Roddenberry and Paramount Pictures agreed on a new canon policy that made Star Trek: The Animated Series non-canonical, as well as removing the canon status of all technical manuals and novels. Gene Roddenberry further stated that some elements from the films Star Trek V: The Final Frontier and Star Trek VI: The Undiscovered Country were "slightly apocryphal". The writers and production staff of Star Trek have also said in interviews and DVD commentaries that they unofficially struck the Star Trek: Voyager episode "Threshold" from canon, and made no references to the events of that episode after it was made; however, being a live-action, televised story, it remains within the general guidelines typically followed by Paramount/CBS Studios to determine canonicity.
The canonicity of the various reference books such as The Star Trek Encyclopedia and various companions accompanying the series are still debated. Many consider such reference works to be canon, while others do not; there is currently no clear answer solving this problem. Much of the information from the The Star Trek Encyclopedia and The Star Trek Chronology comes directly from the television series and movies, which themselves are considered canon, and the books were written by people who worked on the production staff, and sometimes were used as internal references. A similar problem exists with trading cards cataloguing information from the series.
Paramount has made a formal comment on its website about what items are considered canon, calling the concept "fluid". The canonical nature of Star Trek (2009) within established Star Trek continuity, however, remains in doubt. The writers stated in an interview that although the film will be staying true to the Star Trek universe, it will also be a re-imagining (though they have also assured that it will not be changed to the same degree as the second Battlestar Galactica show was from its original). Many fans now believe that the film will attempt to retcon the visual look (and possibly story elements) of Star Trek: The Original Series.
The Star Wars canon is a complex issue, and Lucas Licensing has devised a four-level system called the Holocron continuity database to keep track of the Star Wars canon. The purpose of this database is to chronicle all Star Wars stories, and settle any disputes that may arise within the various productions. The basic rule, however, is that the Star Wars canon comprises the six Star Wars films, along with all officially licensed Star Wars stories not contradicting the films.
The DK Guide to the entire universe utilizes many spin-offs to help describe the six films. Derivative works such as the Star Wars books have aimed to be completely in continuity with each other and with the Star Wars movies.
The Transformers canon is unusual, in that there to date five completely distinct Transformers universes; one consisting of the universe introduced in 1984 with the "Generation One" series and extended through the Beast Wars and Transformers: Universe series, the stand-alone Transformers: Robots in Disguise series, the Unicron Trilogy, consisting of the Armada, Energon, and Cybertron lines, the world featured in the 2007 live-action feature film, and the currently-running Transformers: Animated series. Even within these completely disparate universes, there are multiple stories that contradict each other; for instance, under the banner of "Generation One" are several completely different comic book series from four different publishers, the US cartoon, the Japanese follow-on cartoons which openly replaces the events of the U.S. "Rebirth" miniseries, as well as gamebooks, coloring books, audio cassettes, young adult novels, and video games, as well as the toy packaging itself, all of which take various liberties with the same characters. As such, the Transformers universe includes such problems as multiple origin stories for characters such as the Aerialbots and Optimus Prime. Many fans rectify this by considering the five universes as "continuity families" in which each media (comics, cartoon, toy bios, etc) don't so much as need to coincide with each medium completely as they just have to have some common elements within its respective universe. For instance, the G1 continuity family almost always features Dinobot commander Grimlock as having a unique, caveman-like speech pattern, though the reasons behind this differ between media (primitive technology, effects of a virus, etc...)
The general position of Hasbro, owners of the line, is that all Transformer stories are in fact true, and exist in multiple and separate alternate universes, so that the Marvel Comics stories are just as true as the coloring books, which are as true as the animated shows. All of these universes are connected by the existence of Primus and Unicron, who are singularities; they exist in all of the Transformer universes, even if they are not necessarily referenced in some of them. Primus, for example, was not mentioned at all in the original Transformers cartoon under the "Generation One" banner, but Unicron appeared in the animated movie and in subsequent episodes of the TV series. However, in Beast Wars, Primus was directly referenced by optimus Primal in the season 2 season finale. Beast Wars and its companion series Beast Machines are stated to take place at some point in the future of the cartoon series, though certain elements such as referencing the name of the Autobots spacecraft by the name 'the Ark' are elements introduced in Marvel's comic book as the base was never named in the cartoon.