|See also:||Alternate Universe, Alternate Reality, Fusion, Crossover|
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Crossovers are often an answer to the problem of a much-loved character who lacks a shippable partner in his or her source fandom, or else an answer for a writer who only likes one character of a popular pairing -- like Methos, but not Duncan? Write Methos into another universe altogether! Certain characters seem to lend themselves to showing up in other universes; they are sometimes referred to as Little Black Dresses of fandom.
The presence of the same actor in more than one television show or film that has a fandom can also encourage crossovers. Some actor-based multifandom pairings are popular enough to become their own fandoms; see Kiefer/Lou, Monaboyd, and Hewligan.
The existence of multiple canon sources that share a common universe, and sometimes have crossovers in the canon, gives fans a good start to creating a crossover pairing. The various Star Trek series, the Gateverse, the various CSI shows and many comics fandoms all have canon crossovers and characters who already know each other. Similar canons that are connected in some other way such as a common producer like the Disney Princesses fandom can also encourage crossover pairings as a regular part of the fandom.