Backstory AUs are AUs that take place before the events of Undertale or, in the case of AUs of AUs, before the story of the base-AU, but don't change anything that would happen in Undertale/the base-AU.
The timeframe of a Backstory AU can proceed to continue throughout Undertale's or the base AU's events, as long as it does not conflict with anything that happens in the story.
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