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Most of the time one or the other feels better, but every so often, "which" vs. "what" trips me up.

So, what's the exact difference and when should you use one or the other?

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There's also a very illustration on pearsonlongman.com website I found useful. – asgs Feb 24 at 13:43
up vote 47 down vote accepted

"Which" is more formal when asking a question that requires a choice between a number of items. You can use "What" if you want, though.

Generally speaking, you can replace the usage of "which" with "what" and be OK grammatically. It doesn't always work the other way around, however. There needs to be a context of choice. For example:

Which/What flavor of ice cream do you want?

  • Either is fine, but "which" is better.

Which/What do you want for dessert?

  • "Which" only works in the context of being presented with choices (e.g. a dessert cart right in front of you).
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That's in questions. Which and what are also used in certain types of relative clauses, and their usage is not simple. So this is not a duplicate answer for any new question giving examples with non-question uses of which and what. – John Lawler Feb 18 '15 at 15:45
I've never thought of this being a matter of formal vs informal. Which holds out a promise of a choice between a list of items. What simply demands that a choice be made with no promise of a list. "What do you want, ice cream"? Here you can't use "which". – CandiedOrange Sep 10 '15 at 4:55

Questions of attribute which and what: We usually use which when we are asking about a fixed or limited number of things or people, and what when we are not. Often, however, we can use either which or what with little difference in meaning. Compare:

  • What towns do we go through on the way?

    The speaker doesn't know the area.

  • Which towns do we go through on the way?

    The speaker knows the area and the towns in it.

If you mean when they are conjunctions, it is another question.

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So the answer is that ultimately there are cases where there are no rules but you have to kinda know the connotations? Huh. – Christian Jun 28 '13 at 16:32
"Which are winter months in Europe?" or "What are winter months in Europe"? – Peter May 26 '14 at 11:32
@Peter It would be the former, since it's only a handful of months and you know it all. – asgs Feb 24 at 13:45

protected by RegDwigнt May 20 '12 at 10:11

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