When using commas, follow these rules:
A. Use commas to separate items in a series. If more than two items are listed in a series, they should be separated by commas; the final comma—the one that precedes the word and—is optional. Never use a comma after the word and.
Wrong: My recipe for buttermilk biscuits contains flour, baking soda, salt, shortening, and, buttermilk.
Correct: My recipe for buttermilk biscuits contains flour, baking soda, salt, shortening, and buttermilk.
B. Do not place commas before the first element of a series or after the last element.
Wrong: My investment advisor recommended that I construct a portfolio of, stocks, bonds, commodities futures, and precious metals, to succeed.
Correct: My investment advisor recommended that I construct a portfolio of stocks, bonds, commodities futures, and precious metals to succeed.
C. Use commas to separate two or more adjectives before a noun; do not use a comma after the last adjective in the series.
Wrong: I can’t believe you sat through that long, dull, uninspired, movie three times.
Correct: I can’t believe you sat through that long, dull, uninspired movie three times.
D. Use commas to set off parenthetical clauses and phrases. (A parenthetical expression is one that is not necessary to the main idea of the sentence.)
Correct: Gordon, a writer by profession, bakes an excellent cheesecake.
The main idea is that Gordon bakes an excellent cheesecake. The intervening clause merely serves to identify Gordon; thus, it should be set off with commas.
Correct: The newspaper that has the most insipid editorials is The Daily Times.
Correct: The newspaper, which has the most insipid editorials of any I have read, won numerous awards last week.
In the first of these examples, the clause beginning with that defines which paper the author is discussing—it is necessary to the sentence and should not be set off by commas. In the second example, the main point is that the newspaper won numerous awards, and the intervening clause beginning with which gives additional information.
E. Use commas after introductory participial or prepositional phrases.
Correct: Having watered his petunias every day during the drought, Harold was very disappointed when his garden was destroyed by insects.
Correct: After the banquet, Harold and Martha went dancing.
F. Use commas to separate independent clauses (clauses that could stand alone as complete sentences) connected by coordinate conjunctions and, but, not, yet, for, or, and so. Make sure the comma separates two independent clauses, joined by a conjunction. It is incorrect to use a comma to separate the two parts of a compound verb.
Correct: Susan’s old car has been belching blue smoke from the tailpipe for two weeks, but it has not broken down yet.
Wrong: Barbara went to the grocery store, and bought two quarts of milk.