The serial, Oxford, and Harvard comma
The serial comma (also known as the Oxford comma and the Harvard comma) is the comma used immediately before a grammatical conjunction (usually and, or, and sometimes nor) preceding the final item in a list of three or more items. For example, this three-country list can be punctuated as either Portugal, Spain, and France (with the serial comma) or as Portugal, Spain and France (without the serial comma). To separate words, phrases, or clauses of a series of 3 or more elements, include a comma before the closing and or or. Routine use of the serial comma helps to prevent ambiguity. Graphic Design is all about clear communication and the serial comma helps clarify content. Good designers and copywriters use the serial comma.

The Harvard, Oxford, or serial comma:
     • better matches the spoken cadence of sentences
     • often reduces ambiguity
     • matches practice with other means of separating items in a list (example: when semicolons are used to separate items, a semicolon is consistently included before the last item, even when 'and' or 'or' is present)
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 recommend the use of the serial comma. Most US college writing handbooks advocate use of the serial comma.
Some newspaper style guides recommend not using the serial comma, possibly for economy of space. In the narrow width of a newspaper column this saving counts for more than elsewhere. But here or anywhere one must question whether the advantage outweighs the confusion caused by the omission. Writers use the comma between all members of a series, including the last two, on the common-sense ground that to do so will minimize ambiguities and annoyances at a negligible cost.

Examples

The guests included two strippers, John Kennedy, and Joseph Stalin.

The guests included two strippers, John Kennedy and Joseph Stalin.

A book dedication quoted by Teresa Hayden:
     To my parents, Ayn Rand and God.
There is ambiguity about the writer's parentage - the reader could believe that the writer refers to Ayn Rand and God as her parents. A comma before the and removes the ambiguity:
     To my parents, Ayn Rand, and God.

Here are some types of sandwiches: pastrami, ham, cream cheese and peanut butter and jelly.
According to the two most plausible interpretations of this sentence, four kinds of sandwich are listed. But it is uncertain which are the third and fourth kinds - cream cheese and peanut butter or peanut butter and jelly. Adding a serial comma removes this ambiguity. With a comma after peanut butter, the kinds of sandwich are these:
     • Pastrami
     • Ham
     • Cream cheese and peanut butter
     • Jelly
With a comma after cream cheese, the kinds of sandwich are these:
     • Pastrami
     • Ham
     • Cream cheese
     • Peanut butter and jelly

Other examples:
     • red, white, and blue
     • horses, mules, and cattle
     • height, width, and depth
     • by the bolt, by the yard, or in remnants
     • a, b, and c
     • neither snow, rain, nor sleet
     • I want no ifs, ands, or buts
     • The meal consisted of soup, salad, and macaroni and cheese

The bottom line
• Omitting the final comma may cause ambiguity or confusion.
• Including the final comma will almost never cause ambiguity or confusion.
• Consistency is key. If you're going to use the serial comma once, then use it every time. The serial comma is necessary in some cases, and consistency then requires it be used in other cases.

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