|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.
(…) liking a ship doesn’t necessarily have to come with the belief that those two people are going to get together- just that it would be cool if they did. That’s why Jack Frost/Elsa was a thing. Fans knew that ship was never going to reach harbour, but they still liked the idea of, I dunno, heterossexuality and ice powers, and would draw art and make gifs relating to it. 
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. On the flip side, sometimes fans watch an actor's back catalogue of potentially obscure or fandom-less canons and start writing fanfic of the actor's other roles (see Fannish Drift). Fans of a popular ship can start shipping the actors' other roles together; unless the actors shared screentime before, these pairings are all crossover pairings. Some of these 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 at the time, but as of the early 2020s, fans have been 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 call 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 June 2022, AO3 has 38 canonical Character Combinations tags. The most popular pairings are:
- Bai Yu/Zhu Yilong Character Combinations, with 846 works. The Guardian fandom calls them WeiLan Derivatives or 巍澜衍生.
- Hugh Dancy/Mads Mikkelsen Character Combinations, with 560 works.
- Adam Driver/Domhnall Gleeson Character Combinations, called "Kylux Adjacent" by its fans, with 494 works.
- Marwan Kenzari/Luca Marinelli Character Combinations, also called The Great Kenzarelli Multiverse, with 140 works.
- 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.