Commas with nonessential elements
Certain word groups that modify nouns or pronouns can be restrictive or nonrestrictive—that is, essential or not essential to the meaning of a sentence. These word groups are usually adjective clauses, adjective phrases, or appositives.
Restrictive elements
A restrictive element defines or limits the meaning of the word it modifies; it is therefore essential to the meaning of the sentence and is not set off with commas. If you remove a restrictive modifier from a sentence, the meaning changes significantly, becoming more general than you intended.
restrictive (no commas)
The campers need clothes that are durable.
Scientists who study the earth’s structure are called geologists.
The first sentence does not mean that the campers need clothes in general. The intended meaning is more limited: The campers need durable clothes. The second sentence does not mean that scientists in general are called geologists; only those scientists who specifically study the earth’s structure are called geologists. The italicized word groups are essential and are therefore not set off with commas.
Nonrestrictive elements
A nonrestrictive modifier describes a noun or pronoun whose meaning has already been clearly defined or limited. Because the modifier contains nonessential or parenthetical information, it is set off with commas. If you remove a nonrestrictive element from a sentence, the meaning does not change dramatically. Some meaning may be lost, but the defining characteristics of the person or thing described remain the same.
nonrestrictive (with commas)
The campers need sturdy shoes, which are expensive.
The scientists, who represented eight different universities, met to review applications for the prestigious Belker Award.
In the first sentence, the campers need sturdy shoes, and the shoes happen to be expensive. In the second sentence, the scientists met to review applications for the award; that they represented eight different universities is informative but not critical to the meaning of the sentence. The nonessential information in both sentences is set off with commas.
note: Often it is difficult to tell whether a word group is restrictive or nonrestrictive without seeing it in context and considering the writer’s meaning. Both of the following sentences are grammatically correct, but their meaning is slightly different.
The dessert made with fresh raspberries was delicious.
The dessert, made with fresh raspberries, was delicious.
In the first example, the phrase made with fresh raspberries tells readers which of two or more desserts the writer is referring to. In the example with commas, the phrase merely adds information about one dessert.
Adjective clauses
Adjective clauses are patterned like sentences, containing subjects and verbs, but they function within sentences as modifiers of nouns or pronouns. They always follow the word they modify, usually immediately. Adjective clauses begin with a relative pronoun (who, whom, whose, which, that) or with a relative adverb (where, when).
Nonrestrictive adjective clauses are set off with commas; restrictive adjective clauses are not.
nonrestrictive clause (with commas)
The adjective clause which is located on thirteen acres does not restrict the meaning of Ed’s house; the information is nonessential and is therefore set off with commas.
restrictive clause (no commas)
Because the adjective clause that was born at the San Diego Zoo in 2003 identifies one particular panda out of many, the information is essential and is therefore not set off with commas.
note: Use that only with restrictive (essential) clauses. Many writers prefer to use which only with nonrestrictive (nonessential) clauses, but usage varies.
Adjective phrases
Prepositional or verbal phrases functioning as adjectives may be restrictive or nonrestrictive. Nonrestrictive phrases are set off with commas; restrictive phrases are not.
nonrestrictive phrase (with commas)
The with phrase is nonessential because its purpose is not to specify which of two or more helicopters is being discussed. The phrase is not required for readers to understand the meaning of the sentence.
restrictive phrase (no commas)
Dating from the early 1900s restricts the meaning of newspapers, so the comma should be omitted.
Appositives
An appositive is a noun or noun phrase that renames a nearby noun. Nonrestrictive appositives are set off with commas; restrictive appositives are not.
nonrestrictive appositive (with commas)
Most important restricts the meaning to one book, so the appositive On the Origin of Species is nonrestrictive and should be set off with commas.
restrictive appositive (no commas)
Once they’ve read song, readers still don’t know precisely which song the writer means. The appositive following song restricts its meaning, so the appositive should not be set off with commas.