Comma use with non-restrictive elements

When building upon basic sentences with modifiers – whether with words, phrases, and clauses, we are adding to the meaning and information of the sentence. These modifiers may be either restrictive or non-restrictive, but—except in a few cases—only non-restrictive modifiers are set off by commas.

Missing comma with a non-restrictive element is ranked as #11 on the Top Twenty list of common errors (Lunsford & Lunsford 2008). Interestingly, the related error unnecessary comma with a restrictive element appeared on the 1986 list (Connors & Lunsford).

It’s important to understand the definition of these terms and to be able to identify when a modifier is restrictive or non-restrictive.

A non-restrictive element is considered to be unnecessary to the meaning of the sentence; it is adding nonessential or extra information, which is interesting or clarifying, but does not alter the basic meaning.

A modifier is said to be restrictive when it restricts, or limits, the meaning of a sentence; thus a restrictive element is necessary to the meaning of the sentence’s meaning.

Because they describe conditions that are necessary to the main clause, most adverb clauses are restrictive. However, there are times when an adverb clause may be non-restrictive, such as in the following sentence:

Toni Morrison published her first novel in 1970, when she was thirty-nine.

The adverb clause when she was thirty-nine is not essential to the meaning of the sentence; the information it adds is extra, non-restrictive, since it doesn’t change the meaning if it is left out.

Because non-restrictive elements are nonessential, they are set off by commas. If you leave the comma out, if you are missing a comma with a non-restrictive element, you are incorrectly identifying the element as being restrictive or necessary to the meaning of a sentence.

Let’s compare two more examples.

Restrictive:

Morrison’s novel The Bluest Eye is about an African-American girl who longs for blue eyes.

Non-restrictive:

Her fifth novel, Beloved, won the Pulitzer Prize in 1988.

In the first example, including the title of Morrison’s novel is restrictive or necessary to the meaning of the sentence since it “defines” or identifies which novel is about that particular topic. In the second example, the sentence has already identified that the topic is Morrison’s fifth novel, so including the novel’s title is additional information that is interesting but not required. Because it is non-restrictive, the title Beloved is set off by commas (Little, Brown Handbook 363).

Sometimes the decision about whether to use commas is not obvious because the same clause can be either restrictive or non-restrictive. It depends on that you mean to say.

For example, consider this sentence:

Mr. Crump sold his land which was unprofitable.

If you mean that Mr. Crump sold only the part of his land which was unprofitable, then you would make the clause restrictive by not using a comma and write it as it appears above.

However, if you mean that all of Mr. Crump’s land was unprofitable and that he therefore sold all of it, you would make the clause non-restrictive by using a comma:

Mr. Crump sold his land, which was unprofitable. (from English 3200 402, 204)

As a writer you often have to decide on comma use, whether to add or to omit commas in your sentences. Some comma use is optional, or a matter of organizational or personal style as we’ve discussed in the post on the comma in a series (or the Oxford comma) and the post on commas with introductory word groups. However, you need to be careful not to use them where they are not needed, as with restrictive elements. And you need to make sure that you are not missing commas with non-restrictive elements.

(Resources: Lunsford & Lunsford; Little, Brown Handbook; English 3200)

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