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Why should you use a comma before which in this sentence:

  • The fire, which occurred in 1666, destroyed a large part of central London

Is an example without a comma incorrect?

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I shall try to be succinct, for a fuller explanation, see this answer by StoneyB, as suggested by user178049.

Both with and without commas, your sentence would be grammatically correct, but they would mean slightly different things.

With commas, the clause inside the commas becomes a nonrestrictive relative clause. This means that it is there to add extra, non-essential information to the sentence. The meaning of the sentence doesn't change if you were to remove the clause entirely.

Without commas, the clause becomes a restrictive relative clause. This means that it provides essential information, restricting the noun it modifies to uniquely identify it. Removing the clause would change the meaning of the sentence. (Some people will tell you that you can't use "which" and must use "that" for a restrictive relative clause, but this is not a strict grammar rule used universally.)

So, if you wish to identify which fire by saying "which occurred in 1666", then don't use commas. If you want to add extra information about a fire that has already been identified, then use commas.

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  • Does the restrictive relative clause force the noun being referenced to use a definite article while the non restrictive one doesn't? – user2720402 Jun 21 '17 at 5:16
  • This is a separate question, I would ask it as one (after first doing some research to see whether you can find out the answer for yourself). Comments on an answer to a separate question are a bad place to answer questions! – SteveES Jun 21 '17 at 8:20
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What you are looking at is a dependent relative clause.

If the "which occurred in 1666" part is a nonrestrictive phrase (also an adjective clause) then it requires a comma:

A nonrestrictive phrase adds a little bit of extra (but not essential) information about a noun phrase that you’ve already mentioned in your sentence.

which introduces a nonrestrictive phrase. Therefore, you need a comma before which and another one at the end of the nonrestrictive phrase.

If the same part is a restrictive phrase then it should appear without commas and be an essencial part of the sentence.

https://www.grammarly.com/blog/comma-before-which http://www.cws.illinois.edu/workshop/writers/clauses

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  • I know about restrictive and nonrestrictive phrase. But I have a question. What if two fire incidents(in 1666,1667) had happened, causing the destruction ? You should say it without "comma", shouldn't you? – dz420 Jun 19 '17 at 12:33
  • @dz420 can't really understand what you are referring to! – SovereignSun Jun 19 '17 at 12:41
  • Suppose, a fire occurred in 1966 followed by another in 1997. And both destroyed a large part of central London. If this is the situation, then what should we say, with or without comma? Here I think we should say " The fire which occurred in 1666 destroyed a large part of central London. " – dz420 Jun 19 '17 at 13:32
  • @dz420 If both destroyed then "The fires, which occurred in 1966 and 1977, destroyed a large part of central London." – SovereignSun Jun 19 '17 at 13:34
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    -1 for perpetuating the myth that "which" must introduce nonrestrictive relative clauses see e.g. itre.cis.upenn.edu/~myl/languagelog/archives/000918.html, itre.cis.upenn.edu/~myl/languagelog/archives/001461.html, and to quote someone other then Pullum johnemcintyre.blogspot.cz/2010/03/oh-that.html – DRF Jun 20 '17 at 15:19

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