Concept
A crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders, unofficial efforts by fans, or common corporate ownership. This is different from a spoof, where one discrete character, setting, or universe, copies another character, setting, or universe, often in a comedic manner. Crossovers often occur in an official capacity in order for the intellectual property rights holders to reap the financial reward of combining two or more popular, established properties. In other cases, the crossover can serve to introduce a new concept derivative of an older one. Crossovers generally occur between properties owned by a single holder, but they can, more rarely, involve properties from different holders, provided that the inherent legal obstacles can be overcome. They may also involve using characters that have passed into the public domain with those concurrently under copyright protection. A crossover story may try to explain its own reason for the crossover, such as characters being neighbors (notable examples being the casts from The Golden Girls and Empty Nest) or meeting via dimensional rift or similar phenomenon (a common explanation for science fiction properties that have different owners). Some crossovers are not explained at all. Others are absurd or simply impossible within the fictional setting, and have to be ignored by the series' respective continuities. Still others intentionally make the relations between two or more fictional universes confusing, as with The Simpsons and Futurama, where each show is fiction in the other. In contrast with legal crossovers, unofficial crossovers are created solely because of the artistic pleasure derived by their creators. Unofficial crossovers often take the form of fan-written fiction and fan art, but the trope is increasingly prevalent in amateur films and audio.
