Re: Which and What
What versus which
What is used when specific information is requested from a general or open-ended possible range. Which is used when specific information is requested from a restricted range of possibilities:
A: Iíve got your address. Whatís your phone number?
B: Oh itís 267358.
(NOT Which is your phone number?)
(an open-ended range of possible information)
[looking at a pile of coats]
A: Which is your coat?
B: That black one.
However, where the number of options is shared knowledge among speakers and listeners, what + noun is often used in informal contexts. Here, what is an interrogative pronoun used as a determiner:
[talking about a shop]
What side of the street is it on, left or right?
(or: Which side of the street is it on?)
A: Did you see that documentary about the SARS virus last night?
B: No, what channel was it on?
(or: Which channel was it on?)
by Cambridge Grammar of English, 2006
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