1

I was taught that there is always a comma before "which." If a comma changed the intended meaning then we should use "that" instead.

  1. The resources, which you may find helpful, can be found on the fifth floor of the library.
  2. The resources that you may find helpful can be found on the fifth floor of the library.
  3. The resources which you may find helpful can be found on the fifth floor of the library.

So 1 and 2 would be correct, but never 3.

However, I have often seen "which" without a comma. Is this one of the rules that have changed?

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  • #3 is a re-wording of #2 and has the same meaning; #1 has a quite different meaning. I think you may be right, #3 is incorrect, but I am not sure. Certainly it will be understood.
    – randomhead
    Aug 18 '21 at 18:00
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    All three sentences are correct. 2 and 3 mean exactly the same. In 1 which introduces a restrictive clause and it is the one where the comma is needed. The accepted answer to this question explains it well.
    – None
    Aug 18 '21 at 18:10
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    This happens often. Something which is no more than a guideline for a particular style ends up being expressed as if it were a rule of grammar. Aug 18 '21 at 18:32
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I believe it was Fowler who first explicitly recommended using “that” to introduce restrictive clauses and “which” to introduce descriptive clauses. If you follow that suggestion on style, then normal rules of punctuation will result in a comma before “which” and no comma before “that.”

But that distinction between the usage of “which” and “that” is not a rule of grammar. It is a recommendation about style.

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