A shared universe or shared world is a fictional universe from a set of creative works where one or more writers independently contribute works that can stand alone but fits into the joint development of the storyline, characters, or world of the overall project. It is common in genres like science fiction. It differs from collaborative writing in which multiple artists are working together on the same work and from crossovers where the works and characters are independent except for a single meeting.
Early example: Captain Marvel and Bulletman join forces to battle Captain Nazi, thereby establishing a shared continuity within the Fawcett "universe".
A crossover is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders, common corporate ownership or unofficial efforts by fans.
Comic book crossovers may be traced back to the Golden Age, where characters frequently teamed up on the cover (though far more rarely on the inside). Speed Comics number 32, artwork by Alex Schomburg.
An early example of the comics crossover: Captain Marvel and Bulletman join forces to battle Captain Nazi. Master Comics number 21, artwork by Mac Rayboy.
The Mysterious Island novel is a crossover sequel to Verne's famous Twenty Thousand Leagues Under the Seas (1870) and In Search of the Castaways (1867–68).
Allan Quatermain
