The serial comma comes before the final conjunction in a list. Learn when you must use it.
Here's a listener:
Hi, Grammar Girl. This is Patti from Houston...
Patti and her friends are debating about the serial comma—the comma that comes before the final conjunction in a list. Here's a sentence that uses a serial comma: According to the website Box Office Mojo, the top-grossing movies of all time in the United States are currently Avatar, Titanic, and The Dark Knight.
Do You Always Have to Use Serial Commas?
Although the British are less likely to use serial commas than Americans (1, 2), primarily it's newspapers that allow writers to omit that final comma (1, 3). Newspapers are always looking to save space, and one argument for leaving the comma out is that it's unnecessary in simple sentences. Consider this sentence: When you look at worldwide sales, the top-grossing movies of all time are currently Avatar, Titanic and Lord of the Rings: The Return of the King.
I didn't use a serial comma in that sentence, and there wasn't any confusion. That's the main argument against using the comma in every case—leaving it out often doesn't change anything.
The serial comma is also sometimes called the Oxford comma because it's used by Oxford University Press or the Harvard comma because it's used by Harvard University Press, but I find these names misleading because so many other publications also use the serial comma.
Using Serial Commas Adds Consistency
Although the serial comma isn't always necessary, I favor it because often it does add clarity, and I believe in having a simple, consistent style, instead of trying to decide whether you need something on a case-by-case basis. I also think using the serial comma makes even simple lists easier to read. Really, unless space is incredibly expensive, I can't imagine why anyone would decide the best method is sometimes leave it out and sometimes add it in.*
Always Use Serial Commas to Prevent Confusion
The one thing everyone does seem to agree about is that you have to use the comma when leaving it out would create confusion, as is often the case when the items in the list could have internal conjunctions or are complex in some other way (1, 3, 4, 5, 6, 7, 8). Here’s a sentence that could mean different things with and without the final comma: Rebecca was proud of her new muffin recipes: blueberry, peanut butter and chocolate chip and coconut.
Without a serial comma, you can’t be sure whether the last recipe is a combination of peanut butter and chocolate chip or a combination of chocolate chip and coconut. You can make the meaning clear in two ways: place the final comma after peanut butter or after chocolate chip, or rewrite the sentence so that there is no ambiguity.
If you want to say that the combination is peanut butter and chocolate chip, you can write blueberry, peanut butter and chocolate chip, and coconut, or if you insist on leaving out the serial comma, you can rewrite the list as peanut butter and chocolate chip, coconut and blueberry. But I still think the rewrite is more risky than the sentence with the serial comma because a reader who's just skimming the sentence could be tempted to think that coconut and blueberry is a combination.
Another case where leaving the comma out can be confusing is when the later items in the list can describe an earlier item. An oft-cited example is the made-up book dedication To my parents, Ayn Rand and God. A reasonable reader would assume there are four entities being thanked: mom, dad, Ayn Rand, and God; but without the serial comma you could also conclude that the two parents are Ayn Rand and God. A serial comma clears up any confusion: To my parents, Ayn Rand, and God.
Next: When Even a Serial Comma Isn't Enough