When you use a coordinating conjunction to join two elements of a series, no comma comes before the conjunction. But when you join three or more elements, the preferred rule requires a comma before the coordinating conjunction.
This is called the serial-comma rule. The comma is also known as the Oxford Comma.
Here’s the rule: In a series consisting of three or more elements, separate the elements by commas. When a conjunction (usually and, or, or but) joins the last two elements in the series, put a comma before the conjunction.
Comma Before the and
The serial-comma rule finds impressive support: Henry Fowler follows the rule. Strunk & White follows the rule. The Chicago Manual of Style follows the rule. Bryan Garner follows the rule. Grammar.com follows the rule.
So should you.
As we’ll learn in the eBook Developing a Powerful Writing Style and its section on parallel structure, the series may consist of any grammatical element. You can construct sentences with three or more subjects, verbs, direct objects, objects of prepositions, verbal objects, or any other grammatical part of a sentence.
In the following examples, you’ll find a variety of grammatical elements appearing in a series. Each is named parenthetically after the example.
Examples of the Serial-Comma Rule
The flag is red, white, and blue. (Three predicate adjectives.)
In her will, the woman left jewelry, coins, stocks and bonds, but no cash. (Four direct objects of the transitive verb left.)
The director, the chief, and the chairperson held a confidential meeting. (Three subjects.)
Neither Sun, Maria, nor Fred may authorize this particular expense. (Three subjects.)
The committee reconsidered this issue, found that the supervisor had exceeded her authority, and granted the relief requested by the employee. (Three predicate verbs.)