Coordinating conjunctions join pairs of words, phrases, or clauses, but when such a conjunction is interrupted by an intervening phrase or clause, it can be difficult to know where to put the commas. This is especially true when the conjunction joins the parts of a compound sentence.
Did you notice, for example, the adverbial when clause in the first sentence in this post? That clause comes between the coordinating conjunction but and the independent clause that follows (“it can be difficult . . .”).
The commas in that first sentence follow Chicago style, but even a slight change in the sentence’s structure could require a different treatment relative to the conjunction and the words that immediately follow it. Comma usage may also depend on the style in the surrounding text.
Let’s start by reviewing how to punctuate two common types of compound sentences.
Independent Clauses vs. Compound Predicates
According to CMOS 6.22, a comma normally precedes a coordinating conjunction between two independent clauses—that is, clauses that can stand alone as complete sentences. Coordinating conjunctions include and, but, or, for, nor, so, and yet (a group of words that can be referred to mnemonically—though using a different order—as fanboys).
Note the commas in the following examples:
We activated the alarm, but the intruder was already inside.
Smith’s first book was a runaway hit, and her publisher was demanding a sequel.
But when the conjunction separates a compound predicate, a comma is no longer required (see CMOS 6.23). A compound predicate occurs when the subject of an independent clause is also the subject of a second clause but isn’t repeated—like the subjects “we” and “she” in the following examples:
We activated the alarm but failed to notice that the intruder was already inside.
She wrote two successful books about the environment and became a sought-after speaker.
Had the subjects been repeated, those two examples would have been punctuated like the previous two, with a comma before the conjunction (i.e., “We activated the alarm, but we failed . . .”).
Now we’ll find out what happens when we interrupt those conjunctions.
Interrupted Independent Clauses
Let’s start by interrupting the independent clauses in the first two examples from the previous section. To do that, we’ll insert a dependent clause immediately after each coordinating conjunction:
We activated the alarm, but if I remember correctly, the intruder was already inside.
Smith’s first book was a runaway hit, and before she could catch her breath, her publisher was demanding a sequel.
Commas often come in pairs (see CMOS 6.17), and in each of the examples above, a pair of commas sets off the interrupting clause from the rest of the sentence. But note that the comma before the coordinating conjunction does double duty, allowing us to omit the comma that would usually introduce the dependent clause.*
If you were to retain the omitted comma, it would immediately follow the conjunction, resulting in three commas rather than two. The first comma would precede the coordinating conjunction as it normally does, and the second two would set off the interrupting clause:
We activated the alarm, but, if I remember correctly, the intruder was already inside.
That additional comma (the one after but) would be strictly correct, but it’s unnecessary in Chicago style, which favors a sparer, more economical use of commas.
Interrupted Compound Predicates
Now let’s find out what happens when we interrupt the conjunctions in the examples that feature compound predicates—in this case, by adding a participial phrase (first example) and an adverbial phrase (second example):
We activated the alarm but, still reeling from the effects of carbon monoxide, failed to notice that the intruder was already inside.
She wrote two successful books about the environment and, despite her shyness, became a sought-after speaker.
There were no commas at all in the original versions of these sentences, making these interruptions easier to punctuate than the ones that occurred between independent clauses: simply set them off with a pair of commas. But note that this time the first comma follows rather than precedes the coordinating conjunction.
If the interruption is short enough, you might leave the commas out entirely (see also CMOS 6.31). That could work for the last example, especially if we revise the sentence itself to make it shorter:
She wrote two successful books and despite her shyness became a sought-after speaker.
In most cases, however, setting off the interrupting phrase with commas will be the better option.
A Change of Pace
Back to interrupted independent clauses. Though it’s not Chicago’s primary recommendation, a three-comma approach—in which a comma both precedes and follows the coordinating conjunction—will be appropriate sometimes.
Some authors may simply prefer the extra comma (and expect their editors to retain it). But in creative writing especially, extra commas might be used to slow the reader down. For example, here’s a sentence from “Hammer Attack,” a story by Han Ong published in the January 16 issue of the New Yorker:
To my surprise, I agreed with Gina, and, since we were the only abstainers, Kyung Hee walked us to her home office and shut the door for us.
That’s a quietly climactic moment, when the narrator opts out of watching a video of the attack that’s the subject of the story. Here’s that same sentence in Chicago style (omitting both the comma after and and the optional comma after surprise):
To my surprise I agreed with Gina, and since we were the only abstainers, Kyung Hee walked us to her home office and shut the door for us.
But elsewhere in Ong’s story, extra commas aren’t the rule, as this earlier sentence (among others) shows:
I briefly explained the reading plan, and before she left to take a break she turned serious.
The point is that each of these variations can work. The key is to gain the reader’s trust by adopting a consistent approach while being open—especially in fiction and other creative genres—to context-dependent exceptions. And in Ong’s story, extra commas highlight a key moment for the reader, if only subtly.
Alternatives to Commas
Even if you’re following Chicago style, an occasional third comma between independent clauses might be appropriate in certain cases where extra clarity or emphasis seems warranted. But for a stronger break, parentheses or dashes can be the better option:
We activated the alarm, but (if I remember correctly) the intruder was already inside.
We activated the alarm, but—if I remember correctly—the intruder was already inside.
In that case, only the first comma is retained.
Most of the time, however, Chicago’s two-comma approach should work, and if you can apply that—and maintain consistency—your interruptions should go smoothly.
* Coordinating conjunctions can also begin a sentence. But when that happens (as this sentence illustrates), the rule is the same as for independent clauses, except that a period rather than a comma precedes the conjunction.
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