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[[quoteright:350:[[Creator/MattGroening http://static.tvtropes.org/pmwiki/pub/images/FuturamaSimpsons_628.jpg]]]]
[-[[caption-width-right:350:Drinking brings the [[WesternAnimation/TheSimpsons most awesome]] [[WesternAnimation/{{Futurama}} of characters]] together. [[Recap/TheSimpsonsS26E6Simpsorama (And then it actually happened.)]]]]-]

The classic crossover started itself off as a good way to make the best of what you've got -- so if you have two shows on your roster, it's a no-brainer to have the shows and characters cross over every once in a while, especially if one of the shows is less popular than its sibling.

The popular way of doing this is the "true" crossover, in which a storyline will begin in one series and ''cross over'' into the next one, encouraging viewers to tune into a show that may be thematically similar but which they do not usually watch.

For example ''Series/BuffyTheVampireSlayer'' had a handful of episodes where the characters would head off at the end, only to turn up in ''Series/{{Angel}}'' straight after. This also worked the other way, with a magic amulet in ''Series/{{Angel}}'' turning out to be vitally important for the last-ever episode of ''[[Series/BuffyTheVampireSlayer Buffy]]''. (This also highlights one of the few dangers of crossovers; if you or a fan watched only the ''{{Series/Buffy|TheVampireSlayer}}'' show, but not ''Series/{{Angel}}'', this particular amulet appeared to [[DeusExMachina come totally out of nowhere to save the day]]....) Sometimes this is done to [[PoorlyDisguisedPilot provide the lead-in for a spin-off show]], as happened with both of the ''Series/{{CSI}}'' spin-offs.

Alternatively, a single popular character can cross over from one show into the other, for a brief guest appearance; this has the effect of attracting that character's fans from the other show without requiring the writing teams to sync up or creating DVD- and arc-unfriendly episodes. This is very common in comic books, in which most characters are part of a larger universe, such as the Franchise/MarvelUniverse or Franchise/TheDCU. It happens less often in TV and movie properties based on comic books, since they are often made by different production companies.

As more ways of developing connections between shows formed, we ended up with lots of different subtropes of crossover. All of these can be seen on the CrossoverIndex and any examples for those will be found on their own pages.

[[JustForFun/IThoughtItMeant Not to be confused with]] [[BoringButPractical station wagons made taller]] for ease of acces- sorry, because ADVENTURE!, [[JustForFun/IThoughtThatWas nor with]] the ''Series/StarTrekDeepSpaceNine'' [[Recap/StarTrekDeepSpaceNineS02E23Crossover mirror universe episode]].

* CrossOver/AnimeAndManga
* CrossOver/AsianAnimation
* CrossOver/{{Comics}}
* CrossOver/FanWorks
* CrossOver/{{Films}}
* CrossOver/{{Literature}}
* CrossOver/LiveActionTV
* CrossOver/{{Music}}
* CrossOver/{{Mythology}}
* CrossOver/ProfessionalWrestling
* CrossOver/{{Radio}}
* CrossOver/TabletopGame
* CrossOver/{{Theatre}}
* CrossOver/VideoGames
* CrossOver/VisualNovels
* CrossOver/{{Webcomics}}
* CrossOver/WebOriginal
* CrossOver/WesternAnimation

* Toys/MrPotatoHead has outfits from many other franchises. ''Franchise/{{Transformers}}'' is a given, since Creator/{{Hasbro}} owns both, but the Heads can also dress up as characters from ''Franchise/StarWars'', ''DC'', and ''Marvel''.