Cross Overs occur anytime characters from one work "cross over" into another. Any time characters from one work interact with another work, you have a Crossover. This is extremely common in Fanfic, where the authors take someone else's characters and play with them, often introducing a new setting or new characters in order to better facilitate the story they want to write. This is an index of the varying types of Cross Overs, Rule 50 may apply to some of these.