The Comma

Use a comma to set off the elements of a series (three or more things), including the last two.
Example: My favorite uses of the Internet are sending e-mail, surfing the Web, and using chat rooms.
You may have learned that this comma is not necessary. Sometimes, however, the last two items in your series will glom into one if you don’t use the so-called serial comma.

Use a comma and a coordinating conjunction (and, but, or, nor, for, yet, so) to separate two independent clauses.
Example: The public seems eager for some kind of gun control legislation, but the congress is obviously too timid to enact any truly effective measures.
If the two independent clauses are brief and nicely balanced, this comma may be omitted, but the comma is always correct.

Use a comma to set off introductory elements.
Example: Anxious about the upcoming winter, settlers began to bicker among themselves about supplies.
If the introductory element is brief and the sentence can be read easily without the comma, it can be omitted.

Use a comma to separate coordinate adjectives.
Example: Coaches grew weary of running practices in the drafty, dreary, dilapidated gymnasium.
If you could put a but or an and between the adjectives, you should put a comma between them.

Use a comma to set off elements that express a contrast or a turn in the sentence.
Example: They were looking for something practical, not luxurious.

Use a comma to set off states and countries, years (in a full date), titles, etc.
Example: Their wedding date was set for August 5, 2000, in the college chapel in Newton, Massachusetts.

Use a comma to set off quoted language.
Example: “We can’t see into the future,” said the President, “but we have to prepare for it nonetheless.”

Use a comma to set off parenthetical elements. This is the most difficult rule in comma usage. A parenthetical element is “added information,” something that can be removed from a sentence without changing the essential meaning of that sentence. Deciding what is “added information” and what is essential is sometimes difficult.
Example: Robert Frost, perhaps America’s most beloved poet, died when he was 88.

One last rule: Don’t over-use commas! When a comma is needed, use it; otherwise, do without.