The comma indicates the smallest break in sentence structure of all the punctuation in the English language, but that doesn’t mean it’s always user-friendly. Some people are prone to overpunctuating with commas, crafting staccato sentences that feel too formal or too choppy, while other people write entire paragraphs without a single comma in sight, creating stream of consciousness–type writing that’s equally anxiety-provoking and vague. As with most things, moderation is the ticket where commas are concerned. Today I’m going to tell you how to apply them accurately but economically in a couple of different contexts.

Restrictive and Nonrestrictive Clauses (“Which” vs. “That”)

One of the primary purposes midsentence commas serve is to set off nonrestrictive clauses—clauses that do not alter the essential meaning of a sentence if they are removed, and that are typically introduced by the word “which”:

• My new car, which I bought just last week, is already losing value.
• I went out to dinner with my mother, who loves trying new restaurants.

In the two examples above, if you eliminated either of the italicized phrases within the commas, the sentences’ intended meaning would remain intact. So anytime you introduce a nonrestrictive phrase into a sentence—i.e., anytime you see the word “which”—that’s your cue to surround the phrase in question with commas.

On the other hand, what if we were to replace the nonrestrictive clauses with restrictive clauses—typically introduced by the word “that”?

• The new car that I bought just last week is already losing value.
• I went out to dinner with a woman who loves trying new restaurants.

If we extract the italicized text from these two examples, the reader can’t know when you bought your car or what kind of woman you had dinner with, and thus the basic information the sentences convey changes. So if you see the word “that,” it should be a tip-off not to insert any commas into the sentence.

Restrictive and Nonrestrictive Appositives

Midsentence commas also help readers to understand the significance of names, descriptors, or phrases that serve appositively—i.e., explain the significance of a particular noun. When there is only one of something—for example, if you have only one brother, or if an author has written only one novel—that something is surrounded by commas, as in the following:

• My brother, John, is three years younger than I am.
• Edan Lepucki’s debut novel, California, is a must-read.

Just as removing the commas surrounding nonrestrictive clauses in the first examples does not change the meaning of the sentence, nor does deleting the commas from these two sentences. By the same token, in the following sentences, the absence of commas suggests that more than one of whatever noun (“brother” and “novel”) the writer is using exists:

• My brother John is three years younger than I am.
• Donna Tartt’s novel The Goldfinch is a must-read.

For more information on comma usage, you can’t get better than The Chicago Manual of Style, Chapter 6, which contains all the information you could ever want to know, and probably much more. Read up, and happy punctuating.

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