The basic types of AUs.
Switch Up
Some or all of the canon Undertale characters swap roles and/or personalities with or shift into the roles of the other canon Undertale characters. e.g: Sans replaces Toriel.
If there is an in-universe reason for characters to change roles from their original ones, it doesn't fit in this type either. In that case, the AU is most likely in the Continuity Change and Special Event types.
Crossovers
AUs where the characters of Undertale are replaced by those of a different franchise, such as a show or game (being replaced by a set of OCs also falls under this). If the characters and/or setting of Undertale are placed in a different franchise, Continuity Change - Crossworld should be used instead.
This is NOT meant for crossover stories between AUs.
Basic Change
Main article: Basic Change
Basic Change AUs are AUs that have some fundamental aspect from Undertale or, in the case of AUs of AUs, the base-AU changed in such a way that everything is affected. Whether that be where all the Monsters are instead gods (like Reapertale), where all the Monsters are violent, fell creatures (like Underfell), or where the entire world has been altered from the world of Undertale or the base-AU (like Dreamtale), they are all Basic Change AUs. AUs where all the characters are the same, or the same character from multiple universes take up most of the cast are also Basic Changes.
Story Overhaul
Story Overhaul AUs are AUs that completely change the foundation of the story of Undertale or, in the case of AUs of AUs, the base-AU in such a way that it still clearly resembles its inspiration, while its premise is disconnected from it (like Reapertale).
Theme Overhaul
Theme Overhaul AUs are AUs that completely overhaul the general theme or "feel" of Undertale or, in the case of AUs of AUs, the base-AU. This can either be simply aesthetically, such as Outertale, or alter the characters' personalities, such as Underfell. The base plot is kept mostly unaltered.
World Overhaul
World Overhaul AUs are AUs where the world of Undertale or, in the case of AUs of AUs, the base-AU is completely overhauled to the point that there is barely any resemblence to the world of Undertale or the base-AU.
AU Where Everyone is the Same Person
An AU where everyone is the same person is, as the name suggests, an AU where a large number of characters are the same character. This doesn't just include AUs where all characters are, for example, Undyne (like Undynetale), but also AUs where a bunch of the same character from many different universes are present for the story, and take up a majority of the cast (like Christmas Party).
Continuity Change
Main article: Continuity Change
Continuity Change AUs, or Concept Change AUs, are AUs that modify the continuity of Undertale or, in the case of AUs of AUs, the base-AU in some fashion. Whether the AU takes place before the main story of Undertale, during the same time-period, or after—so long as the AU doesn't qualify for any other AU Type—they're all Continuity Change AUs. AUs that place the world of Undertale into the continuity of another IP (such as EarthBound) are also Continuity Changes.
Backstory
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.
Special Event
Special Event AUs are AUs that change the plot of Undertale or, in the case of AUs of AUs, the base-AU in some fashion, generally through a special event that occurs that otherwise wouldn't happen. Whether this sudden change happens before the main character (Frisk in Undertale) enters the Underground or while the main character is in the Underground, both are Special Events.
Sequel
Sequel AUs are AUs that take place after the main story of Undertale or, in the case of AUs of AUs, after the story of the base-AU.
Crossworld
Crossworld AUs are AUs that place the world of Undertale or, in the case of AUs of AUs, the base-AU (whether it be the characters or the entire setting) into the world of another Intellectual Property (such as EarthBound or Pokémon), or place characters from another Intellectual Property into the world of Undertale.
Self-insert
AUs in which the characters are replaced with real people.
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