Commas with Relative Clauses (which, that, who…)

The Rule:

Use a comma before relative clauses when they introduce a nonrestrictive phrase. Don’t use a comma when they introduce a restrictive phrase.

Terms to Know:

Relative Clauses: Clauses starting with relative pronouns, pronouns that refer to nouns mentioned previously (who, whom, whose, which, that)

Restrictive Phrase: Essential phrase needed to understand the sentence

Nonrestrictive Phrase: Nonessential phrase that adds extra information about a noun phrase

“That” Versus “Which”:

Traditional grammar dictates that you use “that” when the phrase is restrictive and “which” when the phrase is nonrestrictive. In that case, you would always have the comma before “which” and never have the comma before “that.”

While “that” is always used with restrictive clauses, some consider it acceptable now to use “which” with both nonrestrictive and restrictive clauses. However, if your publisher or editor’s style guide dictates that the traditional grammar rule should be adhered to, then so be it.

Examples:

Do you know the neighbor who never showers?

• No comma since the relative clause “who never showers” is restrictive (essential to knowing which neighbor the speaker is referring to)

My mouse problem, which I never dealt with in Arizona, is getting crazy.

• Comma needed since the relative clause “which I never dealt with in Arizona” is nonrestrictive (extra information that isn’t essential to the meaning)

A laptop is a computer which can be moved from room to room.

• No comma since the relative clause “which can be moved from room to room” is restrictive (essential to understanding the description of a laptop)
• Note: If your style guide dictates the “that/which” distinction, you would need to use “that” in this sentence

Can you return the book that you borrowed?

• No comma needed since the relative clause “that you borrowed” is restrictive (essential to knowing which book)

My husband, who isn’t fond of dancing, agreed to three couples dancing lessons.

• Comma needed since the relative clause “who isn’t fond of dancing” is nonrestrictive (just extra information not needed to understand the sentence)

Practice:

1. It made him think of the phone that he had back in high school.
2. My daughter who turns four tomorrow is so funny.
3. The pennies which are part of his great collection are missing.
4. The shoes that she had on yesterday are brand new.
5. The shoes which she had on yesterday are made of real gold.
6. The kids who play with my daughter are a little bossy.
7. I like the collage which hangs in our hallway.

Answers 

1. It made him think of the phone that he had back in high school. (no commas as “that he had back in high school” is telling you which phone and is restrictive/necessary)
2. My daughter, who turns four tomorrow, is so funny.
3. The most expensive stamps, which are part of his great collection, are missing.
4. The shoes that she had on yesterday are brand new. (no comma as “that she had on yesterday” tells you which shoes and is restrictive/necessary)
5. The glittering shoes, which she had on yesterday, are made of real gold.
6. The kids who play with my daughter are a little bossy. (no commas as “who play with my daughter” clarifies which kids and is restrictive/necessary)
7. I like the collage which hangs in our hallway. (no commas. “that” should be switched out for “which” if your style guide follows this distinction)