One use of the comma is to separate elements in a series or inline list. But there are two things one must consider before including or eliminating a serial comma.
1) Is a series involved?
A series is defined as three or more items. A pair of items makes a plural, but plural does not automatically equal series. Also, the presence of a conjunction (and, or, but, etc.) in a sentence does not necessarily mean a comma must be inserted.
Pair: The recipe called for two things Marion couldn't get at her local market: feta cheese and pine nuts.
Because "feta cheese and pine nuts" comprise only two items, no comma is required to separate them.
Series: The obstacle course required the trainees to climb, swim, and jump vertically.
Had there been only two skills listed, no commas would be necessary. Since there are three, they must now be separated. Each additional item that joins this series will demand the same treatment:
The obstacle course required the trainees to climb, swim, jump vertically, demonstrate endurance, apply logic, make precise calculations, and repair a motor using scrapyard material.
2) Which style guide applies?
There are two major schools of thought on the serial comma. One says that each item that appears before the conjunction requires a comma. The Chicago Manual of Style, a guide commonly adopted by manuscript editors, publishers, and many academics, is one of the major manuals that follows this thinking.
The second major philosophy says that a comma immediately before the conjunction is extraneous and should generally be avoided except in cases where a misreading would result. The Associated Press Stylebook is one guide that espouses this rule. The AP Stylebook is the manual used by most newspapers and magazines as well as a large number of advertising firms, public information offices, and many other professionals who regularly work with the media.
So, whether that pre-conjunction comma is necessary in your writing will depend on which style guide has been adopted in whatever organization you write for. Depending on who reads it, either of these sentences could be regarded as correct:
Chicago rule
Come in, sit down, take your shoes off, and make yourself at home.
AP rule
Come in, sit down, take your shoes off and make yourself at home.
The difference between those two sentences is slight, and if you're not reading closely, you may not notice it at all. There are many more substantial differences between the various style guides, but the serial comma rule is one that gives rise to surprisingly heated reactions on both side of the table. For this reason, before beginning a major writing task for any organization, it is a good idea to ask about its style preference.
© 2008 by AnnaLisa Michalski