whichclause

A which clause, in English grammar, is a type of relative clause introduced by the relative pronoun which. It attaches to a preceding noun or noun phrase (the antecedent) and provides additional information about it. Which clauses are commonly used to describe things, rather than people, and they form part of the broader class of relative clauses that also includes that, who, whom, and whose.

Which clauses can function as either restrictive (defining) or non-restrictive (non-defining) elements. In non-restrictive usage, which clauses add extra detail and are set off by commas: The book, which was published in 1999, won several awards. In restrictive usage, the clause specifies the antecedent and does not use a comma: The book which explains the theory is long. In American English, which is often avoided in restrictive clauses in favor of that, although British usage is more flexible.

The pronoun which can occupy various positions inside the clause. It can be the subject of the relative clause, as in The car which stopped suddenly was old, or the object, as in The car which I bought last year is still running well. Which may also appear after a preposition, forming a prepositional relative clause: The room in which I worked was quiet. In informal speech, speakers sometimes replace which with that in restrictive clauses, though which remains standard in non-restrictive clauses.

Overall, which clauses are a primary mechanism for adding descriptive or clarifying information about objects and things in English sentences, functioning alongside other relatives to connect clauses and specify referents. See also relative clauses; English grammar.