Crime fiction is the literary genre that fictionalises crimes, their detection, criminals, and their motives. It is usually distinguished from mainstream fiction and other genres such as historical fiction or science fiction, but the boundaries are indistinct. Crime fiction has multiple sub-genres, including detective fiction (such as the whodunit), courtroom drama, hard-boiled fiction and legal thrillers. Suspense and mystery are key elements that are nearly ubiquitous to the genre.