Serial Commas

The term "serial comma" may sound a little strange, but it's simply the technical term for a comma that comes before the last item in a series or list. The following excerpts were taken from Wikipedia's article entitled "Serial Comma."

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The serial comma (also known as the Oxford comma or Harvard comma) is the comma used immediately before a grammatical conjunction (nearly always and or or) that precedes the last item in a list of three or more items. The phrase "ham, chips, and eggs" is an example that is written with the serial comma, while "ham, chips and eggs" is identical in meaning, but does not include the serial comma.

There is much disagreement about the use of the serial comma. It is nearly standard use in American English, but much less common in British English.

Resolving ambiguity
Use of the serial comma can remove ambiguity.

Consider: "My favorite types of sandwiches are BLT, ham, peanut butter and jam and cream cheese." This is ambiguous because "jam" may be paired with either "peanut butter" or "cream cheese," or all three ingredients can be grouped together. A comma after "jam" can clarify this sentence: "My favorite types of sandwiches are BLT, ham, peanut butter and jam, and cream cheese."

Creating ambiguity
Use of the serial comma can introduce ambiguity, in particular where the last noun phrase but two in the list is singular.

Consider "They went to Oregon with Betty, a maid, and a cook." The presence of the last comma creates the possibility that Betty is a maid, reasonably allowing it to be read either as a list of two people or as a list of three people, context aside. On the other hand, removing the comma leaves the possibility that Betty is both a maid and a cook, so in this case neither the use nor the avoidance of the serial comma resolves the ambiguity.

Usage
The Chicago Manual of Style, Strunk and White's Elements of Style, most authorities on American English and Canadian English, and some authorities on British English—for example, Oxford University Press and Fowler's Modern English Usage—recommend the use of the serial comma. Newspaper style guides—such as those published by The New York Times, the Associated Press, The Times newspaper in the United Kingdom, and the Canadian Press—recommend against it, possibly for economy of space.¹

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You're simply thrilled that I brought up another controversial grammar topic, aren't you? Honestly, I was always taught to use a comma before the last item in a list, but apparently there are many other people who were taught to omit it. No matter what you were taught, keep in mind that most manuals on American and Canadian English recommend using the serial comma to prevent confusion.

According to Wikipedia, even the United States Government Printing Office's Style Manual mandates the use of the serial comma, like in the phrase "red, white, and blue." If it's good enough for government work...

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¹Wikipedia contributors, "Serial comma," Wikipedia, The Free Encyclopedia,
http://en.wikipedia.org/w/index.php?title=Serial_comma&oldid=84032468
(accessed October 29, 2006).