Longman Dictionary of Contemporary English - which

~ 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. )