Because of this, science fiction and fantasy are sometimes collectively referred to as speculative fiction. The abbreviation sff, for "science fiction/fantasy," is also sometimes seen.
Another reason these two genres are often grouped together is that some works combine elements of both: two landmark science fiction series, Star Trek and Space: 1999, both assume the possibility of shape-shifters, for example, without attempting to explain them within their own scientific frameworks. Important figures in speculative fiction sometimes switch between genres: Orson Scott Card and Ursula K. Le Guin, to name two examples, have written major works in both fantasy and science fiction.

