Spotlight on CMOS 6.17
Commas, like the two in this sentence, often come in pairs. When they do, they’re usually acting like miniature parentheses. In fact, whenever you’re tempted to omit the second of two commas, convert them both to parentheses; if your text still reads as intended, keep that second comma.
Commas that don’t work in pairs are usually easy to spot—for example, the ones that separate items in a series. But there are some gray areas, and sometimes you don’t need commas at all. Let’s review.
Parenthetical Commas
As suggested above, when commas can be replaced with parentheses, it’s a good bet that they’re meant to be paired. Such parenthetical commas can set off just about anything, including a word, a phrase, a clause, or a number. Let’s look at some examples.
But first, a common exception.
At the Start/End of a Sentence
Though it goes without saying for most of us, commas differ from parentheses in one important way: Only one is used when setting something off at the beginning or end of a sentence. For example:
Except in this sentence, commas often come in pairs.
Commas, like the two in this sentence, often come in pairs.
Commas often come in pairs, except in this sentence.
Parentheses, by contrast, can pair just about anywhere:
Parentheses come in pairs (wherever they occur).
Now let’s look at some examples of paired commas.
With Parenthetical Clauses
Commas are often used to set off nonrestrictive relative clauses. Such clauses, which provide parenthetical information, typically begin with which (like the one earlier in this sentence) or with who, whom, or whose. Restrictive clauses, by contrast, provide essential rather than parenthetical information and aren’t set off by commas.
Restrictive and nonrestrictive relative clauses are covered in another post (see “Commas with Restrictive and Nonrestrictive Relative Clauses”), so we’ll make this quick. Here’s an example of commas with a nonrestrictive relative clause:
This post, which is about paired commas, features several made-up examples.
Nonrestrictive clauses can generally be set off by parentheses without changing what the sentence means:
This post (which is about paired commas) features several made-up examples.
A restrictive relative clause is typically introduced by that and doesn’t get any commas at all (and parentheses won’t normally make a good substitute):
The post that covers paired commas features several made-up examples.
but not
The post (that covers paired commas) features several made-up examples.
With Parenthetical Words and Phrases
Words and phrases can also be set off by commas, though the rules for using commas aren’t as strict as with relative clauses. When in doubt, try parentheses. If you’re still not quite sure what to do, use commas if it makes sense to treat whatever might be set off as either an aside or as something to be emphasized (see also CMOS 6.51):
This is by far the most comma-heavy document I’ve ever seen.
or, to draw attention to “by far,”
This is, by far, the most comma-heavy document I’ve ever seen.
In some cases, the restrictive/nonrestrictive test that we used for clauses in the previous section will apply to a word or a phrase. To find out how, see “Your Dog[,] Smurf: Understanding Commas with Appositives”—or continue to the next section.
With Dates and States
According to Chicago style (and most other styles), two commas are necessary in the middle of a sentence when setting off a year from a month and day and for setting off the name (or abbreviation) of a state when it follows the name of a city.
Some people prefer to omit that second comma after a year or a state, but the parentheses test suggests that the paired commas are appropriate:
January 22, 1905, was a Sunday.
means
January 22 ([in] 1905) was a Sunday.
and
Madison, Wisconsin, is where The Onion got its start.
means
Madison ([in] Wisconsin) is where The Onion got its start.
If you’re still tempted to leave out that second comma, consider this sentence:
Madison, Wisconsin is where The Onion got its start.
That could mean you’re telling your friend Madison that the state of Wisconsin is where The Onion (the satirical newspaper) got its start. That’s how vocative commas work (see CMOS 6.57).* And though no one would mistake “January 22” for a name (in the date example), the principle still holds.
In fact, not using commas might be preferable to using just one:
Madison Wisconsin isn’t the same as Madison Alabama.
Though most editors would add commas in the example above (three of them), the state names are technically restrictive in that sentence; see “ ‘City, State’: A Comma with Two Competing Roles” for a more detailed explanation. For a similar analysis relative to Jr. and Sr., see “How Strunk Lost His Comma.”
Now for some exceptions.
A Series of Exceptions
As noted above, one exception for paired commas is at the start or end of a sentence. But that’s not an exception so much as a special case. Here are some true exceptions to the rule for commas in pairs.
In a Series
Commas used in a series—including the serial (or Oxford) comma itself—don’t come in pairs; instead, each one works alone:
Apples, oranges, and pears are all popular.
You can tell this is so by applying parentheses:
Apples (oranges) and pears are all popular.
That doesn’t work. That’s because commas in a series simply replace the word and (or or in a series of alternatives). Or at least some of the commas in a series replace and. The serial comma itself—the one before and—is simply a clarifying comma:
Apples and oranges and pears
becomes
Apples, oranges and pears
or, more clearly,
Apples, oranges, and pears.
The serial comma in the last example above—the one before and—organizes the series into three separate elements, each of which gets equal weight. Serial commas may be technically redundant, but they play an important role (see also CMOS 6.19).
With Coordinate Adjectives
Coordinate adjectives constitute another type of series.† For example,
There was a big, beautiful orange on his plate.
not
There was a big, beautiful, orange on his plate.
The comma between “big” and “beautiful,” like commas in other types of series, implies the word and. For more on coordinate adjectives, see CMOS 6.39.
With Timings and Other Dimensions
As suggested above, the commas in a series stand in for the word and (or or). But sometimes these series aren’t obviously series, as with timings:
The winner finished three minutes, forty-two seconds ahead of the pack.
which means
The winner finished three minutes and forty-two seconds ahead of the pack.
Heights and other dimensions work in a similar way:
She was five feet, two inches tall.
which means
She was five feet and two inches tall.
In either case, there’s only one comma. If you’re tempted to add a second comma, try parentheses:
The winner finished three minutes (forty-two seconds) ahead of the pack.
She was five feet (two inches) tall.
Those parentheses, which risk suggesting that three minutes is equivalent to forty-two seconds or that five feet is the same thing as two inches, don’t quite work—so one comma is all that’s needed.
Other Unpaired Commas
Another type of unpaired comma occurs in large numbers, as in 1,432,520, where a comma every third digit makes the number easier to read.
And sometimes a comma can be paired with a different mark of punctuation—for example, in this sentence, where a dash rather than a comma before “for example” helps clarify that “for example” applies to what follows rather than precedes it (see also CMOS 6.54).
Or maybe the comma belongs to the title of a work:
Pixar’s Monsters, Inc. debuted in 2001.
not
Pixar’s Monsters, Inc., debuted in 2001.
Though Chicago would normally omit the comma before “Inc.,” a second comma would be needed in the middle of a sentence when it’s not omitted (see CMOS 6.47). Any comma in a title, however, is ignored relative to the surrounding text (see 6.17).
There are other types of commas that don’t come in pairs (including the comma in this sentence and other commas throughout this post), but most of those should be relatively obvious.
Last Words
When in doubt about adding a second comma, apply the parentheses test. If you could substitute parentheses, as in this sentence, use two commas. Otherwise, one (or none) will usually be the better option.
* When the addressee occurs mid-sentence, vocative commas are also paired: “Did you know, Madison, that Wisconsin is where The Onion got its start?”
† In the manual that gave the Oxford comma its name, the first example of such a comma was in a series of coordinate adjectives (“wise, holy, and energetic”); see “Oxford, Chicago, and the Serial Comma.”
Inverted ducks by Garth Irvine/Wirestock Creators / Adobe Stock.
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