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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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"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
    Commented Feb 18, 2015 at 15:45
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    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".
    – candied_orange
    Commented Sep 10, 2015 at 4:55
  • What about the example of a phone number? There is a fixed set of possible phone numbers, but everyone uses "what" and it would sound unnatural to use "which."
    – Jeff Grimes
    Commented Mar 17, 2017 at 16:26
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    I wouldn't say "Which flavour ice-cream do you want?", unless all the different flavours on offer were available to see, or listed, let's say, on a card - or the possible flavours were known to the person I was asking. If they nor I hadn't any idea what was available, I would say "What flavour do you want?" "Which", in a question, suggests that the available options are known.
    – WS2
    Commented Dec 3, 2017 at 23:27
  • @JeffGrimes we can guess which popular flavor of ice-cream is available even if we don't see them but we cannot really guess an entire phone number, I think that's why we never say "Which phone number is it?".
    – baptx
    Commented Sep 5, 2019 at 16:01
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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
    Commented Jun 28, 2013 at 16:32
  • "Which are winter months in Europe?" or "What are winter months in Europe"?
    – Peter
    Commented May 26, 2014 at 11:32
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    @Peter It would be the former, since it's only a handful of months and you know it all.
    – asgs
    Commented Feb 24, 2016 at 13:45
  • Did any one add that while using which, the preceding clause has a comma after it unlike the case of what.
    – Ram Pillai
    Commented Jan 2, 2020 at 4:44

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