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A crossover is a fanwork in which one universe melts with another universe, or characters from one universe cross over into another universe. An example for a Supernatural/X-Files crossover would be a story in which Sam and Dean help Fox Mulder and Dana Scully investigate a supernatural phenomenon. Alternatively elements from Star Wars, such as the idea of the Force, may be used in a story set in the Supernatural verse.

Supernatural tends to be crossed with fandoms that have similar themes (Whedon-verse, True Blood), share actors (Gilmore Girls, Dark Angel), or are simply popular (Harry Potter, Twilight).