- Use a comma before which when it introduces a nonrestrictive phrase.
- Don’t use a comma before which when it’s part of a prepositional phrase, such as “in which.”
- Don’t use a comma before which when it introduces an indirect question.
Comma Before Which in Nonrestrictive Phrases
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.