which

Which is a word in English that can function as a determiner or as a relative pronoun, and occasionally in interrogative questions. As a determiner, which introduces a noun phrase such as which car or which books. As a relative pronoun, which introduces a relative clause, as in the book which I read. In questions, which asks for a selection from a known set, as in which option should we choose.

In relative clauses, which typically refers to things or animals rather than people (who, whom, or that are more common for people). It can introduce restrictive clauses, where the information identifies a specific entity, or nonrestrictive clauses, which add extra information and are set off by commas, as in The report, which was published yesterday, was well received. In American English, that is often preferred for restrictive clauses, while which is common in nonrestrictive clauses; British usage is more flexible but often uses that for restrictive clauses as well.

In questions, which is used when the set of possible answers is known or limited. It contrasts with what, which asks about a broader, more open set of possibilities. Examples include Which color do you prefer? Which of these books did you read? Some contexts also use whichever as a compound determiner or pronoun, extending the idea of choice to a broader range.

Etymology traces which to Old English and Proto-Germanic roots, related to other Germanic languages. See also that, who, whom, and which forms the family of wh- words used for asking questions and introducing relative clauses.