Updated October 21, 2025
Spotlight on CMOS 6.28 and 6.35
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 two independent clauses or a compound predicate.
Did you notice, for example, the dependent when clause in the first sentence in this post (“when such a conjunction . . .”)? 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 how the sentence is worded 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 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 (words that can be referred to mnemonically, though using a different order, as fanboys).
Note the commas before the conjunctions 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 joins the two parts of a compound predicate instead of two independent clauses, a comma is no longer required (see CMOS 6.24). 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 . . .”; “She wrote two successful books about the environment, and she became . . .”).
Now we’ll find out what happens when we add interruptions.
Interrupted Independent Clauses
To interrupt the independent clauses in the first two examples from the previous section, 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 that omitted comma, there would be 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—and it may be appropriate either for emphasis or in a work that favors a closely punctuated style (see CMOS 6.16)—but it’s generally unnecessary in Chicago style, which favors a sparer, more economical use of commas.
The words that introduce the dependent clauses in the examples above—if and before—are called subordinating conjunctions. You can think of those interrupting clauses, then, as setting up two consecutive conjunctions—coordinating followed by subordinating (but if; and before). See also CMOS 5.206.
Interrupted Compound Predicates
Let’s find out what happens when we interrupt 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 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, you might leave the commas out entirely (see also CMOS 6.34). That could work for the last example, especially if we revise the sentence 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.
Alternatives to Commas
As noted above, an occasional third comma between independent clauses might be appropriate in certain cases (as for emphasis or in a closely punctuated work). But for a more noticeable break, parentheses or dashes are also an 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.
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.
Switching Things Around
The sections above show two main scenarios: (a) independent clauses interrupted by a dependent clause and (b) a compound predicate interrupted by an adverbial or participial phrase.
But sometimes it’s the other way around. When that happens, the rules are the same. For example, here’s a sentence with two independent clauses interrupted by a participial phrase:
We activated the alarm, but still reeling from the effects of carbon monoxide, we failed to check the doors.
or, in more closely punctuated prose,
We activated the alarm, but, still reeling from the effects of carbon monoxide, we failed to check the doors.
And here’s a sentence with a compound predicate interrupted by a dependent clause:
We activated the alarm but, if I remember correctly, failed to check the doors.†
Key Points
When a sentence is interrupted by a dependent clause or an adverbial or participial phrase, the interrupting clause or phrase is usually set off by commas. Between two independent clauses joined by a coordinating conjunction, however, the comma that might normally follow the conjunction is usually omitted in Chicago style.
* Coordinating conjunctions can also begin a sentence. But when that happens (as this sentence illustrates), the rule is the same as between independent clauses (i.e., the comma after the coordinating conjunction can usually be omitted).
† In the similar case of a dependent clause that comes between an independent clause and a dependent clause introduced by a subordinating conjunction like that, any comma between the two subordinating conjunctions (e.g., that if) can usually be omitted: “We decided that if the alarm went off, we’d leave the house.” See CMOS 6.28.
Déconnexion (Disconnected), by DURIS Guillaume / Adobe Stock.
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