What exactly do they do—and what is a serial comma?
Commas have two different purposes: one is the typical piece of punctuation’s job—giving a sentence rhythm, helping the reader know where to breathe, rest, and make sense of the whole. The other is grammatical: like dashes and parentheses (brackets, if you’re in the U.K.), commas can help define the exact meaning of a word or phrase within a sentence.
So commas, used like these ones right here, show where to slow down or pause—plus, as we all know, they’re for making lists. That’s where the serial comma comes in. In a list of three or more things—knife, fork, and spoon, for instance—you can put in a comma after the word that comes right before and—or not: knife, fork and spoon. It’s mostly a style thing: the New York Times doesn’t use the serial comma, though many publications do. Occasionally it helps prevent confusion in a list whose components already contain and—for instance: I spent the day at Crate and Barrel, Bed Bath and Beyond, and West Elm, and all I have to show for it is a knife, fork, and spoon set.
But commas also have a grammatical role. Look at this:
My son Mickey and my daughter, Margaret, are wonderful kids in their different ways.
The commas—or lack of them—are actually supplying more information about the speaker’s family than you might suspect. Why is there a pair of commas for the daughter and not for the son? Because the commas are a device to show limits—they define. They tell the reader that there’s only one daughter—Margaret—in this family. The lack of commas around Mickey, on the other hand, means that son is not defining—in other words, this parent has more than one son. Somewhere, Mickey and Margaret have an unnamed brother (or brothers—the commas can’t tell us how many there may be).