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When there are multiple "official" works or original media, the question of what is and what is not canonical can be unclear. This is resolved either by explicitly excluding certain media from the status of canon (as in the case of Star Wars), by assigning different levels of canonicity to different media (as was in the case of Star Wars before the franchise was purchased by Disney), by considering different but licensed media treatments official within their own continuities but not across them or not resolved at all. The use of canon is of particular importance with regard to reboots or re-imaginings of established franchises, such as the 2009 Star Trek film, because of the ways in which it influences the viewer experience.
The Star Wars canon originally existed on several levels. The highest level was the original Star Wars films, and statements by George Lucas; tie-in fiction from the Star Wars expanded universe had a different level of canonicity. The complex system was maintained by Leland Chee, a Lucasfilm employee.
After Disney bought the franchise, all material published prior to April 25, 2014 that was not any of the Star Wars movies or the CGI cartoon The Clone Wars was declared "Legends" continuity, marking them as no longer official canon. All subsequent material exists on the same level of canon, with the Lucasfilm Story Group being established to ensure no contradictions among canon works.
Disney acquisition, canon revision and Star Wars Legends
On October 30, 2012, Lucasfilm was sold to The Walt Disney Company for $4.05 billion. After the acquisition, Disney and Lucasfilm established the Lucasfilm Story Group, a committee whose job is to keep track of and define the "canon" in an effort to unify the films, comics, and other media with the existing canon.
On April 25, 2014, Lucasfilm and Disney removed all Expanded Universe works from the canon. They announced that the existing seven films and The Clone Wars television series are the "immovable objects" of Star Wars storytelling. Previously published material has been relabeled under a "Legends" label, and future content will present a different vision of people, places and events after Return of the Jedi. They also announced that all future Star Wars stories will be connected and on equal canon level as the films, with guidance coming from the Star Wars story group. Additionally, it was announced that the films of the sequel trilogy will not follow the same story as that of the Expanded Universe works taking place after the events of Return of the Jedi.
The first official canon novel in the revised continuity was Star Wars: A New Dawn by John Jackson Miller, which is a prequel novel to Star Wars Rebels. It was released September 2, 2014.
Determining canonicity (Original canon)
The Star Wars canon was first defined in a 1994 interview with Lucas Licensing's Sue Rostoni and Allan Kausch in issue #23 of the Star Wars Insider:.
Gospel, or canon as we refer to it, includes the screenplays, the films, the radio dramas and the novelizations. These works spin out of George Lucas' original stories, the rest are written by other writers. However, between us, we've read everything, and much of it is taken into account in the overall continuity. The entire catalog of published works comprises a vast history—with many off-shoots, variations and tangents—like any other well-developed mythology.
This policy has been further refined and fleshed out over the years. The official Star Wars website has also detailed the role of canon, expanded universe, or "EU" sources, and how they fit into overall Star Wars continuity. In a 2001 "Ask the Jedi Council" response by Steve Sansweet (director of fan relations) and Chris Cerasi (an editor for LucasBooks at the time), it was stated that:
When it comes to absolute canon, the real story of Star Wars, you must turn to the films themselves—and only the films. Even novelizations are interpretations of the film, and while they are largely true to George Lucas' vision (he works quite closely with the novel authors), the method in which they are written does allow for some minor differences. The novelizations are written concurrently with the film's production, so variations in detail do creep in from time to time. Nonetheless, they should be regarded as very accurate depictions of the fictional Star Wars movies.
The further one branches away from the movies, the more interpretation and speculation come into play. LucasBooks works diligently to keep the continuing Star Wars expanded universe cohesive and uniform, but stylistically, there is always room for variation. Not all artists draw Luke Skywalker the same way. Not all writers define the character in the same fashion. The particular attributes of individual media also come into play. A comic book interpretation of an event will likely have less dialogue or different pacing than a novel version. A video game has to take an interactive approach that favors gameplay. So too must card and roleplaying games ascribe certain characteristics to characters and events in order to make them playable.
The analogy is that every piece of published Star Wars fiction is a window into the 'real' Star Wars universe. Some windows are a bit foggier than others. Some are decidedly abstract. But each contains a nugget of truth to them.
By 1996, Licensing kept an in-house bible of reference materials as the volume of publications, facts, and figures grew to such unwieldy proportions that it became difficult to know everything relevant to a particular project. They finally decided something had to be done to organize the increasingly large collection of media which chronicled the Star Wars universe. A system of canon was developed that organized the materials into what was and wasn't fit for the Star Wars story.
In 2000, Lucas Licensing appointed Leland Chee to create a continuity tracking database referred to as the "Holocron".
The Holocron was divided into five levels (in order of precedence): G-canon, T-canon, C-canon, S-canon, and N-canon. These levels are no longer in use after April 25, 2014.
G-canon was George Lucas canon: Considered absolute canon, it included Episodes I–VI (the released films at that time), and any statements by George Lucas (including unpublished production notes from him or his production department that are never seen by the public). Elements originating with Lucas in the scripts, filmed deleted scenes, film novelizations, reference books, radio plays, and other primary sources were also G-canon when not in contradiction with the released films. G-canon overrided the lower levels of canon when there was a contradiction.
T-canon was Television canon: referred to the canon level comprising only the animated film Star Wars: The Clone Wars and the two television series Star Wars: The Clone Wars and Star Wars Rebels. Its precedence over C-Level canon was confirmed by Chee.
C-canon was Continuity canon: consisting of materials from the Star Wars expanded universe including books, comics, and games bearing the label of Star Wars. Games and RPG sourcebooks were a special case; the stories and general background information were themselves fully C-canon, but the other elements such as character/item statistics and gameplay were, with few exceptions, N-canon.
S-canon was Secondary canon: covering the same medium as C-canon, it was immediately superseded by anything in higher levels of canon in any place where two elements contradicted each other. The non-contradicting elements were still a canon part of the Star Wars universe. This included certain elements of a few N-canon stories.
N-canon was Non-canon: "What-if" stories (such as stories published under the Star Wars: Infinities label), crossover appearances (such as the Star Wars character appearances in Soulcalibur IV), game statistics, and anything else directly contradicted by higher canon ends up here. N-canon was the only level that was not considered official canon by Lucasfilm. Any published material that contradicted things established in G-canon and T-canon was considered N-canon.
So to sum it up - The Offical Cannon is the Star Wars movies and their immediate novels. As of April 25 2014 all works excluding the movies and Clone Wars tv show are now categorized as Star Wars Legends. All future Star Wars stories will be considered on equal canon level with the films.
On April 25, 2014, Lucasfilm officially revised and solidified the canon, stating that all previously released Expanded Universe works would be re-branded under the new Star Wars Legends banner, in order to ensure a flowing timeline with the release of the Star Wars sequel trilogy. They also announced that future Star Wars stories will be considered on equal canon level with the films, with guidance coming from the Star Wars story group.