Commas confuse many, maybe most, people. One comma use that catches many writers is the comma with but also.
First, let’s take a look at how but also is used. Then we’ll take a look at commas with but also, or, more specifically, the lack of commas. Finally, we’ll take a look at a case where we use the comma with but also.
Not Only . . . But Also
The phrase not only . . . but also is a correlative pair. Two common correlative pairs are either . . . or and neither . . . nor. Together, the two parts of a correlative are required for the phrase to be complete. They are used as a set. This means, briefly, that if you use not only, you must also use but also.
No Comma with But Also
As with all correlative pairs, we don’t use a comma before the second part of the pair. For example, the following sentence does not need a comma before but also.
He was sure not only that the harvester was full but also that he was too tired to empty it.
As you can see from this correct example, we have no comma before but also. This follows Zen Comma Rule AI: Don’t use a comma to separate the two halves of a correlative pair. The reason for this is fairly simple. Because the second part is necessary to complete the expression, it needs to remain connected to the first part.
Commas separate elements in a sentence. We don’t want to separate the two parts, so we don’t use a comma before but also.
Exception to the Rule
As with many punctuation rules, there are exceptions to this rule. Sometimes, an expression or phrase between not only and but also requires commas. Let’s add such a phrase to our example to understand what this means.
He was sure not only that the harvester was full, which was expected at the end of the day, but also that he was too tired to empty it.
Sure enough, we see a comma before but also. However, that comma is not there because of but also. Rather it is there to match the first comma before which.
The clause which was expected at the end of the day needs to be separated from the sentence with commas because, like all which clauses, it provides information that is not necessary to understand the main point of the sentence. This comma use is reflected in Zen Comma Rule J: Use commas to separate non-restrictive phrases and clauses starting with which.
Thus, the commas are required by the which clause and not the not only . . . but also correlative pair. In this case, that clause happens to be immediately prior to but also, so the comma it requires is immediately before but also.
If we take out the which clause and the two commas it requires, we are left with no comma before but also.
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