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, Buffy the Vampire Slayer has a handful of episodes where the characters head off at the end, only to turn up in Angel straight after. This also works the other way, with a magic amulet in Angel turning out to be vitally important for the last-ever episode of Buffy. This also highlights one of the dangers of crossovers; if you or a fan watched only Buffy the Vampire Slayer, but not Angel, this particular amulet appeared to come totally out of nowhere to save the day... (See Red Skies Crossover.)
Alternatively, a single popular character can cross over from one show into an 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 Marvel Universe or The DCU. It happens less often in TV and movie properties based on comic books, since they are often made by different production companies.
Examples of crossovers in media:
- Anime & Manga
- Comic Books
- Fan Works
- Live-Action TV
- Professional Wrestling
- Tabletop Games
- Tokusatsu (encompasses both Film & Live-Action TV)
- Video Games
- Web Original
- Western Animation
- Other Media
List of crossover tropes:
Crossover stories (and plot devices)
- Adaptation Amalgamation: A crossover that's an adaptation of both/all the stories being crossed over.
- Common Crossover: When two particular works have a frequent tendency to be crossed over in Fan Fiction.
- Crossover Finale: The final installment of the story is also a crossover with another work.
- Fully Absorbed Finale: When a show's finale isn't grand, because it's wrapped up in an episode of a different show set in the same universe.
- Crossover Punchline: A crossover that's just a brief joke.
- Doppelgänger Crossover: A story that crosses two works which both feature the same actor playing different characters.
- Fusion Fic: A fanfic where the characters in one story replace the characters in another.
- Massive Multiplayer Crossover: A crossover between more than two works (often many works).
- Crossover Cosmology: A crossover between different religious mythologies (e.g. Thor hanging out with Zeus).
- Deconstruction Crossover: Lots of different stories in one genre are crossed over in order to deconstruct (and/or reconstruct) the genre.
- Fairy Tale Free-for-All: A crossover between a lot of different Fairy Tales.
- Mascot Fighter: A Fighting Game featuring iconic characters from various franchises.
- Mascot Racer: A Racing Game featuring iconic characters from various franchises.
- Monster Mash: Popular monsters, villains, and other characters from classic horror fiction (e.g. Dracula and Frankenstein's Monster) hanging out with one another.
- Obvious Crossover Method: A story has an obvious method through which crossovers can occur.
- Poorly Disguised Pilot: The pilot episode of a spin-off show is technically an episode of the original show it's being spun-off from.
- Required Spinoff Crossover: A spin-off show does a crossover episode with its parent show.
- Spiritual Crossover: A crossover that uses a bunch of Captain Ersatz characters instead of the originals.
- Team-Up Series: A crossover involving heroes teaming up to face their collective enemies.
- Vacation Crossover: A crossover wherein characters meet because one or both parties are on vacation, since they normally live in different places.
- Weird Crossover: When two vastly different works of fiction are crossed over, resulting in a very (humorously) bizarre combination.
- Story-Breaker Team-Up: When the crossover doesn't work very well because of how the two stories are TOO different from each other.
Crossover settings (and canon/continuity)
- Intercontinuity Crossover: A crossover between two different stories that are not (normally) set in the same universe.
- Shared Universe: Multiple works of fiction are set in the same overall continuity, which allows for occasional (fully canon) crossovers.
- Bat Family Crossover: A crossover that only affects people in one immediate in-universe location.
- Canon Welding: An author takes two previously-unrelated works they made and fuses them together into one universe.
- Crisis Crossover: A crossover between all the characters and settings within a Shared Universe.
- Cross Through: Similar to a Crisis Crossover, but without intra-universe character interaction; a storyline starts like an installment of one story, then switches between different settings within that universe.
- Red Skies Crossover: Similar to a Crisis Crossover, but while all the works within a universe are affected, the characters don't all meet each other.
- Intra-Franchise Crossover: A crossover between different adaptations/continuities/universes within the exact same franchise (often as part of a shared multiverse).
- Modular Franchise: Two media franchises created by the same company merge into one.
- The World as Myth: The idea that every work of fiction ever created exists in one big multiverse, with each fictional universe making up one dimension; possibly even claiming that the real world is part of this omniverse.
- Character Overlap: When certain characters have a tendency to do the crossing over.
- Crossover Couple: Two characters from different works fall in love.
- Crossover-Exclusive Villain: An antagonist exclusively created for a crossover, (usually) with no relation to any previous canon.
- Crossover Combo Villain: When a crossover creates a new villain by combining elements from both stories.
- Crossover Villain-in-Chief: When universes collide, one villain rises up to lead the others.
- Guest Fighter: A character from a different work shows up inexplicably in an unrelated video game, usually a Fighting Game hence the title.
- Lawyer-Friendly Cameo: It's legal to have a crossover without permission, if you make it so that technically they're a different character, or you censor the character a bit.
- Original Generation: A crossover between lots of different works, with a new character as the main protagonist.
- Public Domain Character: Fictional characters who are currently owned by nobody often tend to freely appear in many different works of fiction.
- Sidelined Protagonist Crossover: When crossovers don't revolve around the main hero from one or more of the involved stories, or demotes them to supporting characters.
- Sir Cameos-a-Lot: A character who appears in more cameos and crossovers than they do in their own original works.
- Spotlight-Stealing Crossover: In a Massive Multiplayer Crossover, one character gets more focus than the rest of them.
- Transplant: A character leaves one story/setting to join another.
- Rogues Gallery Transplant: A villain introduced in one work crosses over with another, and is more associated with that one.
- Ultimate Showdown of Ultimate Destiny: An epic crossover battle which pits two or more characters originating from different stories against each other.
- Fight Dracula: A duel between the famous vampire Dracula and a hero from another story.
- Hypothetical Fight Debate: People have a discussion revolving around this question: "Who would win in a fight between [these two different characters]?"
- Let's You and Him Fight: For some reason, crossovers often have heroes from different stories violently clashing with each other upon their first meeting.
- Power Creep, Power Seep: Trouble arises in a crossover battle when one character (or group of characters) is just way too powerful compared to the other side.
- Versus Title: Many crossover works have "versus"/"vs." in the title, which is meant to imply a fight happening between both of the titular characters/franchises.