In English-language punctuation, a serial comma (also called a series comma, Oxford comma, or Harvard comma) is a comma placed immediately after the penultimate term (i.e., before the coordinating conjunction, such as and or or) in a series of three or more terms. For example, a list of three countries might be punctuated as either "France, Italy and Spain" (without the serial comma) or "France, Spain" (with the serial comma).
Opinions among writers and editors differ on whether to use the serial comma, and usage also differs somewhat between regional varieties of English. British English allows constructions with or without this comma, whereas in American English the comma is common and sometimes even considered mandatory by some style guides. The APA style, The Chicago Manual of Style, Garner's Modern American Usage, The MLA Style Manual, Strunk and White's The Elements of Style, and the U.S. Government Printing Office Style Manual recommend or mandate it. By contrast, the Associated Press Stylebook and The New York Times Style Book advise against it. In Canada, the stylebook published by The Canadian Press advises against it. Most British style guides do not mandate its use. The Economist Style Guide notes that most British writers use it only where necessary to avoid ambiguity. A few British style guides mandate it, most notably The Oxford Style Manual (hence the name, "Oxford comma"). However, the University of Oxford Style Guide (2014) advises against its use.
The Oxford Companion to the English Language notes: "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."
There are cases in which the use of the serial comma can avoid ambiguity and also instances in which its use can introduce ambiguity.
History
The comma itself is widely attributed to Aldus Manutius, a 15th-century Italian printer who used a mark now recognized as a comma to separate words. Etymologically, the word comma, which became widely used to describe Manutius's mark, comes from the Greek koptein (literally "to cut off"). The Oxford comma (defined above) has been used for centuries in a variety of languages, though not necessarily in a standardized or prescribed manner. The Oxford comma as we know it is most often attributed to Horace Hart. The printer and controller of the Oxford University Press from 1893 to 1915, Hart authored the eponymous Hart's Rules for Compositors and Readers in 1905 as a style guide for the employees working at the press. The guide called for the use of the serial comma (hence the name "Oxford Comma"), but the punctuation mark had no distinct name until 1978, when Peter Sutcliffe referred to the Oxford comma as such in his book about the history of the Oxford University Press.
Sutcliffe, however, attributed the Oxford comma not to Horace Hart but to F. Howard Collins, who mentioned it in his 1905 book, Author & Printer: A Guide for Authors, Editors, Printers, Correctors of the Press, Compositors, and Typists.
Arguments for and against
Common arguments for consistent use of the serial comma: Use of the comma is consistent with the conventional practice of the region. It can resolve ambiguity (see examples below). Its use is consistent with other means of separating items in a list (for example, when semicolons are used to separate items, one is always included before the last item).
Common arguments against consistent use of the serial comma: Omission of the comma is consistent with the conventional practice of the region. It can introduce ambiguity (see examples below). When space is at a premium, the comma adds unnecessary bulk to the text.
Many sources are against both systematic use and systematic avoidance of the serial comma, making recommendations in a more nuanced way (see Recommendations by style guides and subsequent sections).