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 affects how readers 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 the second sentence, however, the clause who have been convicted of drunken driving is not essential to the meaning because it merely provides more information about what it modifies, The two drivers involved in the accident. Therefore, the clause is set off with commas.

To decide whether an element is restrictive or nonrestrictive, read the sentence without the element, and see if the deletion changes the meaning of the rest of the sentence. If the deletion does change the meaning, the element is probably restrictive, and you should not set it off with commas. If it does not change the meaning, the element is probably nonrestrictive and requires commas.

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.

– John McMurtry, “Kill ’Em! Crush ’Em! Eat ’Em Raw!”

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

The who clause is necessary to the meaning because only the man who rescued the puppy won the gratitude; therefore, the clause takes no commas.

– Ernest Hemingway, A Moveable Feast

The clause describing Shakespeare and Company is not necessary to the meaning of the sentence and therefore is set off with a comma.

In general, set off an adverb clause that follows a main clause only if it begins with although, even though, while, or another subordinating conjunction expressing contrast.

The clause while she prefers periods and short sentences expresses contrast; therefore, it is set off with a comma.

Do not set off any other adverb clause that follows a main clause.

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 are set off with commas (31l).

The phrase in poor health does not limit the meaning of Frédéric Chopin and so is set off with commas.

Appositives

An appositive renames a nearby noun (31l). When an appositive is not essential to identify what it renames, it is set off with commas.

Jon Stewart‘s name identifies him; the appositive the comic and news commentator provides extra information.

The appositive is restrictive because Mozart wrote more than one opera.