Serial comma

The serial comma, also known as an Oxford comma or a Harvard comma, is a comma used before the word "and" or "or" in a list of three or more items. For example, the phrase "ham, chips, and eggs" is written with a serial comma, but "ham, chips and eggs" is not.

Most authorities on modern American English usage recommend using the serial comma. The AP Stylebook is the most prominent exception. Certain newspapers, such as the New York Times, omit the serial comma (supposedly to save space). In British and Australian English, the serial comma is normally not used except when its absence produces ambiguity. The alternate names for the serial comma come from Oxford University Press and Ayn Rand and God". A comma before the "and" would rule out the unlikely possibility that the writer's parents are Ayn Rand and God. Another ambiguity arises in any list where the last or penultimate item contains the word "and". For example, the instruction, "At the store, buy bread, macaroni and cheese, and bacon", would be ambiguous without the serial comma before "and bacon".

However, consistent use of the serial comma can also introduce ambiguity, as in the phrase "We considered Ms. Roberts for the roles of Marjorie, David's mother, and Louise", which is ambiguous as to whether Marjorie is David's mother, but would not be if the final comma were removed. Thus, whichever convention is followed, the writer may be forced to use the alternative (or to rephrase the sentence) to remove ambiguity in particular cases.