Similar to Alternate Histories, these are where multiple universes are posited as existing side by side, either branching off from each other, or having a completely separate existence. Unlike Alternate History stories, however, there is an assumption that one can travel between them both ways. The best examples of this that I own are the Crosstime novels from Andre Norton where a civilisation had to develop crosstiming in order to survive the devastating world war that had reduced it to a radio active slag heap. The requirement of Alternate Histories to have some realistic basis for a change is not so urgent in this type of stories.
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