What is "serial comma"

n. A comma preceding the final conjunction in a list of three or more items, especially to provide clarity.

In English language punctuation, a serial comma or series comma (also called Oxford comma and Harvard comma) is a comma placed immediately before the coordinating conjunction (usually and or or) in a series of three or more terms. For example, a list of three countries might be punctuated either as "France, Italy,and Spain" (with the serial comma), or as "France, Italyand Spain" (without the serial comma).

Opinions among writers and editors differ on whether to use the serial comma. In American English, a majority of style guides mandate use of the serial comma, including APA style, The Chicago Manual of Style, The MLA Style Manual, Strunk and White's Elements of Style, and the U.S. Government Printing Office Style Manual. In contrast, the Associated Press Stylebook and the stylebook published by The Canadian Press for journalistic writing advise against it. It is used less often in British English, but some British style guides require it, including The Oxford Style Manual. According to The Oxford Companion to the English Language, "Commas are used to separate items in a list or sequence ... Usage varies as to the inclusion of a comma before and in the last item ... This practice is controversial and is known as the serial comma or Oxford comma, because it is part of the house style of Oxford University Press." Some use it only where necessary to avoid ambiguity, in contrast to such guides as Garner's Modern American Usage, which advocate its routine use to avoid ambiguity.