Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Use commas to set off nonrestrictive (nonessential) elements, but not restrictive (essential) elements
The comma
Punctuation

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 (essential) 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 (nonessential) 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, some meaning may be lost, but the defining characteristics of the person or thing remain the same.

NONRESTRICTIVE (WITH COMMAS)

✵ The campers need sturdy shoes, which are expensive.

✵ The computer scientists, who represented eight different universities, met to review applications for the Turing Award.

In the first sentence, the campers need sturdy shoes, and the shoes happen to be expensive. In the second sentence, the computer 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 the dessert.

Adjective clauses

Adjective clauses are patterned like sentences, but they function within sentences as modifiers of nouns or pronouns. They usually follow the word they modify and begin with a relative pronoun (who, whom, whose, which, that) or with a relative adverb (where, when). (See also 49e.)

Nonrestrictive adjective clauses are set off with commas; restrictive adjective clauses are not.

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.

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.

EXERCISE 33-3

Add or delete commas where necessary in the following sentences. If a sentence is correct, write “correct” after it. Answers appear in the back of the book.

a. Choreographer Alvin Ailey’s best-known work Revelations is more than just a crowd-pleaser.

b. Twyla Tharp’s contemporary ballet Push Comes to Shove was made famous by the Russian dancer Baryshnikov. [Tharp has written more than one contemporary ballet.]

c. The glass sculptor sifting through hot red sand explained her technique to the other glassmakers. [There is more than one glass sculptor.]

d. A member of an organization, that provides job training for teens, was also appointed to the education commission.

e. Brian Eno who began his career as a rock musician turned to meditative compositions in the late 1970s.