Here are the general rules for when to use a comma with which:
- Place a comma before which if it is introducing a nonrestrictive phrase.
- Do not place a comma before which if it is part of a prepositional phrase like “in which.”
- Do not place a comma before which if it introduces an indirect question.
Nonrestrictive phrases add extra, nonessential information about a noun phrase that was already mentioned in the sentence.
Celena’s new refrigerator, which is less than a week old, has started making a buzzing sound.
In this sentence, which introduces a nonrestrictive phrase. So, you need a comma both before which and at the end of the nonrestrictive phrase. This is a nonrestrictive phrase because you can take it out of the sentence, and the meaning doesn’t change.
Celena’s new refrigerator has started making a buzzing sound.
Restrictive phrases can’t be taken out of the sentence without changing the meaning. Usually, restrictive phrases are introduced by that instead of which.
Refrigerators that Celena buys always seem to have problems.
The highlighted phrase is restrictive. If you take it out of the sentence, the meaning changes: Refrigerators always seem to have problems. You’re not talking about refrigerators in general; you’re specifically talking about the refrigerators that Celena buys. That means the phrase is restrictive and does not need commas.
Examples of sentences that need a comma before which:
My new shoes, which were quite expensive, hurt my feet.
Her nonprofit organization, which she’d dreamed of for years, is finally starting.
Sally picked up the bag, which had fallen on the floor.
Prepositional Phrases with Which
Don’t add a comma before which if it is part of a prepositional phrase like of which, in which, or on which.
I watched three movies, the shortest of which lasted one hour.
The box in which the gift arrived was full of holes.
The premise on which the book was based was far-fetched.
Using Which in Questions
Do not add a comma before which if it introduces a question.
She asked Paul which bus she should take.
Related: For more punctuation knowledge, check out our”Punctuation Pointers” section here!