A crossover is a fanfic or other fan-created work in which two or more fandoms are combined in some way. Crossovers are an old story trope of fanfiction; as soon as fans started writing stories in multiple fandoms, they started crossing those sources together. In story notes, a crossover is frequently notated with an 'x', so a Star Trek story crossed over with Star Wars is sometimes written as ST/SW and sometimes written ST x SW.
Crossovers can be a love/hate topic among fans. Some fans adore the possibilities they bring, while others believe they are rarely written well - or are simply uninterested in reading about the characters in any fandom except the one they originate from. Fans will frequently make rec lists for well-written crossovers, or else post requests asking for crossover works from particular fandoms that they'd like to see combined.
Crossovers have also been the subject of numerous communities and dedicated events (such as fests or exchanges), so that crossover lovers can connect and share the works that they love to create and consume.
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