Rule
Use commas to set off nonrestrictive elements (extra information), but do not use commas with restrictive elements (necessary information).
Terms to Know
Restrictive: A word, phrase, or clause that is essential to the meaning of the sentence.
Nonrestrictive: A word, phrase, or clause that can be removed without changing the basic meaning of the sentence.
Examples
My sister, who lives in Seattle, is coming to visit.
→“Who lives in Seattle” is extra information (nonrestrictive), so it is surrounded by commas. Unless I had more than one sister, and that was identifying which sister.
The students who studied passed the test.
→“Who studied” is necessary to identify which students, so no commas are used.
The red car, dented on one side, sped away.
→“Dented on one side” is not essential, so it takes commas.
Anyone arriving late will not be admitted.
→“Arriving late” is necessary to know who is affected, so no comma is used.
The author, known for her thrillers, just released a children’s book.
→“Known for her thrillers” is extra information—commas are needed.
Books that are overdue must be returned immediately.
→“That are overdue” is essential to know which books—no commas.
My uncle who lives in Boston is a firefighter.
→“Who lives in Boston” is necessary to identify which uncle
My car, which I bought last year, already needs new brakes.
→“Which I bought last year” is nonessential—commas are needed.
→The guests sitting near the window ordered dessert.
“Sitting near the window” tells us which guests—no commas.
Practice
My neighbor who just had a baby is exhausted.
That movie which won three awards is overrated.
My brother a talented musician just released his first album.
Employees working overtime must report to HR.
The cat curled on the couch looks cozy.
His laptop having broken in two needed replacing.
The woman wearing the green scarf waved at us.
Ms. Gonzalez who teaches chemistry is retiring.
Answers
My neighbor who just had a baby is exhausted. (restrictive – no commas)
That movie, which won three awards, is overrated. (nonrestrictive – commas)
My brother, a talented musician, just released his first album. (nonrestrictive – commas)
Employees working overtime must report to HR. (restrictive – no commas)
The cat curled on the couch looks cozy. (restrictive – no commas)
His laptop, having broken in two, needed replacing. (nonrestrictive – commas)
The woman wearing the green scarf waved at us. (restrictive – no commas)
Gonzalez, who teaches chemistry, is retiring. (nonrestrictive – commas)