The serial comma, also called the Oxford comma or the Harvard comma, is the comma before the conjunction that comes in front of the last item in a series of items.
We went to the laundry, the grocer, the hardware store, and the hair salon.
It became fashionable a few decades ago for high school English teachers to counsel their young charges that the serial comma is redundant because the conjunction alone—usually and—is sufficient to separate the final two items. They've been doing it ever since. This seems reasonable at first glance but doesn't bear close scrutiny. Without the final comma, the last two items of a serial group are not distinguished from each other as emphatically as are the other items in the group, and this carries a potential for confusion in certain constructions.
The biggest inspirations in my life have been my parents, John Kerry, and Hilary Clinton.
The sentence above takes on a whole new meaning when you omit the final comma:
The biggest inspirations in my life have been my parents, John Kerry and Hilary Clinton.
One very good “rule” in writing, whether it's fiction or nonfiction, is to be consistent in your stylistic choices. All that means is that, when you have a choice between more than one acceptable way of doing something, you should do it the same way every time—pick a style and stick with it. If you write the possessive “James's” in one place, don't spell it without the final ess—“James'”—somewhere else. If you write “advisor” with an “o” over here, don't write “adviser” with an “e” over there. Be consistent. Now, you already know you can't completely omit the final serial comma from your writing; you have to use it whenever necessary to avoid confusion. When you don't use it you save nothing more than a single comma, hardly a huge gain in conciseness. Since the use of the final serial comma is always considered correct and is endorsed by the highest authorities, including the ultimate “big gun” of such things, The Chicago Manual of Style, your better choice is to be consistent and always use it and never have to worry about when you need it. There's no downside.
This blog entry is reprinted from the section "Certain Matters of Form and Usage" in Writers' Devils: The Grammar Guide for Fiction Writers, by Dan Persinger, available in Kindle eBook format at Amazon.