The series comma is the comma that precedes the conjunction (and, or, but) in a list of items. For example, popular professional team sports in the U.S. include football, baseball, ice hockey, and basketball. The purpose of the comma in the list is to help the reader separate the items from each other. Even if the reader knows nothing of sports, the reader knows that “ice hockey” go together because there’s no comma between “ice” and “hockey”.
Not everyone, however, agrees on always using the series comma. For example, the style guidelines used by Associated Press writers and other journalists direct that a series comma should not be used in a simple series. On the other hand, styles guides such as Oxford University Press, Chicago Manual of Style, and Strunk direct writers to use the comma after each item in the list except the last item (the one that appears after the and). In fact, the series comma is sometimes called “the Oxford comma”, because Oxford has been recommending its use for over 100 years.
I generally recommend using the series comma because doing so reduces the chance of inadvertently introducing confusion or misinformation. A good friend of mine often uses the following example:I have colored marbles in my pocket. I have a red, white and blue marble. Do I have two marbles or three?
Since a series comma wasn’t used the grammatically correct answer would be two: one red and one both white and blue. But in writing where the series comma is often omitted, the reader may answer three: one red, one white, and one blue. If the sentence had been written, “I have a red, white, and blue marble” there would be no confusion on whether there are two or three marbles.If you care, share.