Many usage guides provide guidance on when to use commas and when to omit them. Some commas are needed to prevent misreading, some are harmful, and others are optional. Since the mid–twentieth century, usage experts, such as Theodore M. Bernstein, have advocated using “a minimum of commas” (359). Claire Kehrwald Cook warns, “Commas call attention to words. They make readers pause and take notice. Unless you want that effect, don’t use commas that the sentence structure doesn’t require” (125). In other words, use commas only when they help make your meaning clear.
So when does a sentence have too many commas, and when does it have too few? Take the following example:
What time are we eating mother?
If mother is on the menu, then this sentence requires no commas. If, however, the question is addressed to mother, then a comma should be added before “mother”:
What time are we eating, mother?
Here’s another example:
This evening, the moon came out, and, as usual, the owls hooted.
You could include all these commas, but they might distract your reader. Since any of the commas could be omitted without changing the meaning of the sentence, you might pare down the punctuation in any one of a number of ways, depending on what you wish to emphasize:
This evening the moon came out, and as usual the owls hooted.
This evening, the moon came out and as usual the owls hooted.
This evening the moon came out, and, as usual, the owls hooted.
When the absence of commas might cause misreading, it is best to revise. The following sentence contains several necessary commas:
This evening the moon came out, but, as I explained later, I didn’t hear the owls hooting, because I went to bed before they appeared.
The first comma is necessary to separate two long independent clauses joined by but, the next two are needed to set off the inessential clause as I explained later, and the last comma is needed to make clear that the reason the speaker didn’t hear the owls hooting was because the speaker went to bed before they appeared and not, say, because the speaker was wearing earplugs.
To pare down the commas, you might revise as follows:
This evening the moon came out, but—as I explained later—I didn’t hear the owls hooting, because I went to bed before they appeared.
This evening the moon came out. As I explained later, I didn’t hear the owls hooting, because I went to bed before they appeared.
When it comes to commas, usage guides can help you with the rules, but ultimately you should use comma sense.
Bernstein, Theodore M. The Careful Writer: A Modern Guide to English Usage. Atheneum, 1965.
Cook, Claire Kehrwald. Line by Line: How to Edit Your Own Writing. Houghton Mifflin, 1985.