Should I use a comma before "which"?

To understand if you should use a comma before which, we need to understand the difference between a restrictive and a non-restrictive clause.

A restrictive clause is one where its removal would alter the meaning of the sentence. It is necessary for understanding the meaning of the sentence. In the US, many style guides suggest that you should use "that" rather than "which" for restrictive clauses, e.g. The fruit that we bought was tasty.

A non-restrictive clause does not alter the meaning of the sentence. This additional information is used with “which” and a pair of commas placed before and after the clause:

• Correct: The fruit, which everyone found tasty, was my best idea.
• Incorrect: The fruit which everyone found tasty was my best idea.
• Incorrect: The fruit, which everyone found tasty was my best idea.
• Incorrect: The fruit which everyone found tasty, was my best idea.

Which can also appear as part of a prepositional phrase, e.g. The team in which we played was great. When "which" appears in a prepositional phrase, it should not be preceded by a comma. Other examples of which in a prepositional phrase are "on which" and "of which."

• Correct: The games, the longest of which lasted two hours, were fun.
• Incorrect: The games, the longest of, which lasted two hours, were fun.

Should you use a comma before "which" in an indirect question?

When you're using "which" at the start of an indirect question, it should be preceded by a comma.

• Correct: I asked, which is the best?
• Incorrect: I asked which is the best?

This is similar to the rule about using a comma before a quotation as you can imagine the indirect question being surrounded by quotation marks.

Restrictive and non-restrictive clauses both modify their antecedents, but they have a very important difference.

A restrictive clause, also called an essential clause, is necessary to the meaning of the sentence. It cannot be removed without changing the meaning of the entire clause. On the other hand, a non-restrictive clause can be taken away without changing the meaning of the sentence. This is why it’s also called a non-essential clause.

Both restrictive and non-restrictive clauses can start with the word which. This makes it hard to know how to punctuate these clauses. Only non-restrictive clauses need to be set apart by commas. Think of it as denoting which part of the sentence is extra and can be removed. Restrictive clauses do not need commas.

Restrictive Clauses

Restrictive clauses provide important information and help their antecedents be more specific. Here’s an example that uses who instead of which.

• The man who visited me was my father.

If we remove the restrictive clause (who visited me), we no longer know which man we are talking about.

• The man was my father.

In these situations, a comma is not necessary. In American English, we often use that instead of which for restrictive clauses.

Non-Restrictive Clauses

Non-restrictive clauses give more information, but they are not necessary for the sentence to make sense. They provide extra details. Non-restrictive clauses use which and should be set apart by commas.

• Correct: Everyone who read the book, which was recommended by Julie, enjoyed it.
  
• Incorrect: Everyone who read the book which was recommended by Julie enjoyed it.

The sentence still makes sense if we remove the non-restrictive clause (which was recommended by Julie) because we know that we are talking about the book everyone read.

• Correct: I drank the tea, which I had gotten for Christmas, and curled up with a good book.
  
• Incorrect: I drank the tea which I had gotten for Christmas and curled up with a good book.