comma with nonrestrictive clauses

The commas that set off a nonrestrictive clause, and the absence of commas around a restrictive clause.

A nonrestrictive clause adds information about a person or thing the sentence has already identified, and commas set the clause off from the rest of the sentence: Our new office, which opened in March, has free parking. A restrictive clause identifies which person or thing the sentence means, and it takes no commas: The office that opened in March has free parking.

Commas around a clause can change the meaning of a sentence. My sister, who lives in Denver, is getting married means that the speaker has one sister. My sister who lives in Denver is getting married means that the speaker has more than one sister and identifies the one who lives in Denver.

A nonrestrictive clause follows the noun it describes, so it never begins a sentence. In the middle of a sentence, it takes a comma at each end: The museum, which reopened last spring, now charges admission. At the end of a sentence, it takes one comma, before the clause: The tour ended at the old courthouse, which now houses a library.

That never introduces a nonrestrictive clause. A nonrestrictive clause about a thing begins with which, and one about a person begins with who or whom. Whose, where, and when can also begin a nonrestrictive clause: We moved to Austin, where housing cost less.

Examples

Use Paul’s car, which he bought last year, needs new brakes. (A clause that adds information about a noun already identified, such as Paul’s car, takes commas.)

Use Students who miss the exam must schedule a makeup. (A clause that identifies which people or things the sentence means takes no commas.)

Avoid Students, who miss the exam, must schedule a makeup. (The commas around who miss the exam are incorrect, because they make the sentence say that all students miss the exam. A clause that identifies which people the sentence means takes no commas. Write Students who miss the exam must schedule a makeup.)

Avoid Lake Michigan, which borders four states is the only Great Lake entirely within the United States. (A comma must follow states, because a nonrestrictive clause in the middle of a sentence takes a comma at each end. Write Lake Michigan, which borders four states, is the only Great Lake entirely within the United States.)

Avoid Our house, that we bought in 2019, needs a new roof. (That is incorrect, because that never introduces a nonrestrictive clause. Write Our house, which we bought in 2019, needs a new roof.)