To my knowledge, we do not put comma before which but I have seen some sentences putting comma before which.

i.e. I don't like stories which have unhappy endings.

i.e. The heavy use of automobiles in urban areas, which could lead to a serious air pollution problem in cities.

Why the former one doesn't have comma before which but the latter one does?

closed as unclear what you're asking by Janus Bahs Jacquet, TrevorD, jimm101, TaliesinMerlin, Skooba Mar 12 at 15:54

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  • You might be interested in the Comma Guide by The Oxford Dictionaries, part of their Punctuation Guide. – Weather Vane Mar 8 at 20:13
  • Unrelated to your question, but your second example isn't a grammatically correct sentence. It's a sentence fragment. There is no predicate. "The heavy use of automobiles in urban areas" needs to do something. To make it a sentence, just remove "which": The heavy use of automobiles in urban areas could lead to a serious air pollution problem in cities. – only_pro Mar 8 at 20:43
  • Where did you get the idea that "we do not put comma before which"? That is an incorrect assumption. Sometimes you can put commas before which and sometimes you shouldn't! Also, you Q. may be more suited to our sister site [English Language Learners](ell.stackexchange.com/]. – TrevorD Mar 11 at 0:08

"... which have unhappy endings" modifies "stories" and thus does not take a comma.

"... which could lead to a serious..." is a new clause (I think!) and takes a comma -- and avoids the possibility of it being incorrectly understood as modifying "use."

  • I think that's right, but just in the most ordinary interpretation. If you wanted to say that all stories have unhappy endings, you'd need a comma. – Greg Lee Mar 8 at 21:21

First, note that your second sentence is ungrammatical. You need to add something else to it:

The heavy use of automobiles in urban areas, which could lead to a serious air pollution problem in cities, is prohibited.

In US English, your first sentence would likely use that, not which:

I don't like stories that have unhappy endings.

Or it could dispense with both words:

I don't like stories with unhappy endings.

Only in UK English is it traditionally considered acceptable to use which with a restrictive clause if that would also work. So, in UK English, you may or may not see a comma before which. Either is acceptable, so long as the presence or lack of comma is acceptable. (I don't know enough about this UK styling to talk more about how the particular word is chosen.)

In US English, however, if either of the words sounds like it could work, then it's normally that without a comma or which with a comma.

Still, regardless of the word itself, the US and UK have the same general guideline: use a comma before a nonrestrictive clause but not before a restrictive clause.

There are situations where you would use which without a comma, even in US English:

✘ I don't know that item to choose.
→ ✔ I don't know which item to choose.

That of those houses do you live in?
→ ✔ Which of those houses do you live in?

  • It’s perfectly acceptable to use which non-restrictively in AmE as well – it’s just more common in BrE. StoneyB (who is American) wrote an ELU blog post about it once, advocating for using which more. (In your examples at the end, which is an entirely different word, a relative determiner and interrogative pronoun, respectively. Since_that_ does not exist as a either of those at all, it stands to reason that that is not an option there.) – Janus Bahs Jacquet Mar 9 at 11:43
  • @JanusBahsJacquet In my answer, I said that (in the US) which is normally nonrestrictive and that is normally restrictive. When I mentioned the UK, I said that which can also be used restrictively, which is not normally the case in the US. Also, in my additional comment, which is not a different word. It's the same word being used in a different sense or grammatical role. – Jason Bassford Mar 9 at 18:16
  • Gah, that should have been restrictively, not non-restrictively in my previous comment. My point was that though it’s more common in BrE, it’s not traditionally considered unacceptable in AmE. – Janus Bahs Jacquet Mar 9 at 18:22
  • 1
    @JanusBahsJacquet Got it. I've heard that it is unacceptable in US law specifically. (Even if that is domain specific.) And according to Oxford Dictionaries (with respect to the restrictive which), "this common British construction is not strictly incorrect in American English, but it is generally avoided, especially in formal writing." – Jason Bassford Mar 9 at 21:44

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