which

 
Pronunciation: /wɪtʃ/

interrogative pronoun & determiner

  • asking for information specifying one or more people or things from a definite set: [as pronoun]: which are the best varieties of grapes for long keeping? which of the suspects murdered him? [as determiner]: which way is the wind blowing?

relative pronoun & determiner

  • used referring to something previously mentioned when introducing a clause giving further information: [as pronoun]: a conference in Vienna which ended on Friday [after preposition]: it was a crisis for which he was totally unprepared [as determiner, after preposition]: your claim ought to succeed, in which case the damages will be substantial

Phrases

which is which

used to convey that it is difficult to distinguish between two or more people or things: there is no confusion as to which is which

Origin:

Old English hwilc, from the Germanic bases of who and alike

On the differences between which and that in relative clauses, see that (usage).