|See also:||Alternate Universe, Alternate Reality, Crossover, Fusion, Same-Actor 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 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.
Actor-based crossover pairings
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, DarkHawk, and Hewligan.
Kiefer/Lou became popular in the late 1980s and was apparently an outlier, but as of the 2020s, fans are writing fic for a number of these ships. The terminology used to describe this phenomenon varies. For example, Hannibal fans who write fanfiction about the other roles played by Mads Mikkelson and Hugh Dancy were calling it "Hannibal Extended Universe" or "Hannigram rarepairs". They tangled with AO3 tag wranglers, who provided the canonical, but less used formatting Hugh Dancy/Mads Mikkelsen Character Combinations.
As of October 2021, AO3 has 28 canonical Character Combinations tags. The most popular one is Bai Yu/Zhu Yilong Character Combinations, with 801 works. The Guardian fandom calls them WeiLan Derivatives or 巍澜衍生. The second most popular is Hugh Dancy/Mads Mikkelsen Character Combinations, and the third is Adam Driver/Domhnall Gleeson Character Combinations, called "Kylux Adjacent" by its fans.
- See Category:Crossover Relationships for more examples.
Controversies and criticism
The idea of crossover pairings as a whole has generated some controversy from those who prefer pairings between characters of the same fandom. Some fans support crossover pairings because they think it makes sense, while others oppose it and believe that it gets in the way of their preferred pairings, especially if what they prefer is canon. Even though crossover pairings usually don't affect franchises, it is still not uncommon for it to cause wars between supporters and haters.