which

/wItS/ determiner, pronoun

1 used to ask or state what people or things you mean when a choice has to be made: Which of these books is yours? | Ask him which one he wants. | Karen comes from either Los Angeles or San Francisco, I can't remember which.

2 used to show what specific thing or things you mean: Did you see the letter which came today? | This is the book which I told you about.

3 used especially in written language after a comma, to add more information about a specific thing or things, or about the first part of the sentence: The train, which takes only two hours, is quicker than the bus, which takes three. | The police arrived, after which the situation became calmer. | in which case (=used to talk about a situation that you have just mentioned): She may have missed the train, in which case she won't arrive for another hour.

4 which is which used to say that you cannot tell the difference between two very similar people or things: They look so alike it's difficult to tell which is which.

USAGE NOTE: WHICH FORMALITY As subject of a relative clause which restricts the meaning of a noun, that is used more often than which in informal English: The street market which/that is held near my house. In informal or spoken English, you can often leave out that or which. For example, you are likely to say: Did you get the things you wanted? rather than: Did you get the things that/which you wanted? The form to which is very formal: He would lunch in one of the clubs to which be belonged. You would more usually say: ... one of the clubs (that) he belonged to In relative clauses that add information but do not restrict the meaning of what comes before, you usually use which, especially after a comma: He's always really rude, which is why people tend to avoid him.