|See also:||Pastfic, Futurefic, Story Tropes|
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Backstory is a term used to describe all the plot and character details that make up a character, that occurred before the main timeline of the work. In original media, backstory is often revealed in flashback scenes or in entire episodes of TV series devoted to pre-canon events.
In fanworks, backstory is often invented by the fan to flesh out characters that may have been too minor to be fully explored in canon, or deliberately left mysterious by the original creators. Fanfiction exploring backstory is sometimes pastfic or it may be that the backstory is used to flesh out characterization or plot in a canon setting.
Backstory is also the focus of some Time Travel fics; with character(s) travelling to the past and encountering a younger version of themselves or a friend/colleague/enemy. These works sometimes led to surprising revelations about a character's past, that may explain their actions in canon, or demonstrate their character's development.
Backstory from canon can be used in AUs to add a sense of familiarity to the characters who have been taken out of their usual milieu, or can become so commonly used as a motivating device in stories that it comes to seem almost like fanon. In SGA fanworks, Rodney McKay's run-in with a heartless piano teacher, a moment mentioned in passing in the SG1 episode, Redemption, part 2,  led to many stories focused around his thwarted musical talent, both canon and AU.
A very common trope in fanworks is to create backstories of childhood experiences to explain canon character behaviour. The backstory that fandom created for General Hux, following his appearance in Star Wars: The Force Awakens is one example of this. The character was often portrayed as been the illegitimate son of an abusive father, suffering abuses at home and at the First Order Academy. Interestingly much of this fanon backstory was confirmed with the release of novels set in the Star Wars universe.
In most cases, fan creating backstory for a character in an open canon runs the risk of being Jossed or Kripked by later events. For example, many BtVS fans created fanon backstories for the character Spike, that were based in no small part on a line of dialogue implying that Angel was Spike's sire. The episode "Fool for Love" jossed the majority of Spike's fanon backstory, including the identity of his sire. The character's canon backstory led to much fan discussion and many revisions to fanon, with Spike's revised backstory quickly appearing in fics further exploring his human life and his siring.