Nonrestrictive elements are word groups that do not limit, or restrict, the meaning of the noun or pronoun they modify. Setting nonrestrictive elements off with commas shows your readers that the information is not essential to the meaning of the sentence. Restrictive elements, on the other hand, are essential to meaning and should not be set off with commas. The same sentence may mean different things with and without the commas:

The bus drivers rejecting the management offer remained on strike.

The bus drivers, rejecting the management offer, remained on strike.

The first sentence says that only some bus drivers, the ones rejecting the offer, remained on strike. The second says that all the drivers did.

Since the decision to include or omit commas influences how readers will interpret your sentence, you should think especially carefully about what you mean and use commas (or omit them) accordingly.

In the preceding sentence, the clause who have been convicted of drunken driving is essential because it explains that only drivers who have been convicted of drunken driving should lose their licenses. Therefore, it is not set off with commas.

In this sentence, however, who have been convicted of drunken driving is nonrestrictive because it merely provides additional information about the particular drivers who were involved in the accident. Therefore, the clause is set off with commas.

To decide whether an element is restrictive or nonrestrictive, mentally delete the element, and see if the deletion changes the meaning of the rest of the sentence. If the deletion does change the meaning, you should probably not set the element off with commas. If it does not change the meaning, the element probably requires commas.

Using commas with adjective and adverb clauses

An adjective clause that begins with that is always restrictive; do not set it off with commas. An adjective clause beginning with which may be either restrictive or nonrestrictive; however, some writers prefer to use which only for nonrestrictive clauses, which they set off with commas. (See 37e.)

RESTRICTIVE CLAUSES

The claim that men like seriously to battle one another to some sort of finish is a myth.

—JOHN McMURTRY, “Kill ’Em! Crush ’Em! Eat ’Em Raw!”

The adjective clause is necessary to the meaning because it explains which claim is a myth; therefore, the clause is not set off with commas.

The adjective clause is necessary to the meaning because it identifies the man, so it takes no commas.

NONRESTRICTIVE CLAUSES

I borrowed books from the rental library of Shakespeare and Company, which was the library and bookstore of Sylvia Beach at 12 rue de l’Odeon.

—ERNEST HEMINGWAY, A Moveable Feast

The adjective clause is not necessary to the meaning of the independent clause and therefore is set off with a comma.

An adverb clause that follows a main clause does not usually require a comma to set it off unless the adverb clause expresses contrast.

The adverb clause expresses contrast; therefore, it is set off with a comma.

Using commas with phrases

Participial phrases may be restrictive or nonrestrictive. Prepositional phrases are usually restrictive, but sometimes they are not essential to the meaning of a sentence and thus are set off with commas.

NONRESTRICTIVE PHRASES

Using commas around the participial phrase makes it nonrestrictive, telling us that all of the singer’s children interrupted.

RESTRICTIVE PHRASES

Wood cut from living trees does not burn as well as dead wood.

The participial phrase cut from living trees is essential to the meaning.

The bodyguards were the men in dark suits and matching ties.

The prepositional phrase in dark suits and matching ties is essential to the meaning.

Using commas with appositives

An appositive is a noun or noun phrase that renames a nearby noun in a sentence. When an appositive is not essential to identify what it renames, it is set off with commas.

NONRESTRICTIVE APPOSITIVES

Savion Glover’s name identifies him; the appositive the award-winning dancer provides extra information.

RESTRICTIVE APPOSITIVES

The phrase is restrictive because Mozart wrote more than one opera. Therefore, it is not set off with commas.