- Separates the clauses of a compound sentence connected by a coordinating conjunction: A difference exists between the musical works of Handel and Haydn, and it is a difference worth noting. The comma may be omitted in short compound sentences: I heard what you said and I am furious. I got out of the car and I walked and walked.
- Separates and or or from the final item in a series of three or more (optional): Red, yellow, and blue may be mixed to produce all colors.
- Separates two or more adjectives modifying the same noun if and could be used between them without altering the meaning: a solid, heavy gait. But: a polished mahogany dresser.
- Sets off nonrestrictive clauses or phrases (i.e., those that if eliminated would not affect the meaning of the sentences): The burglar, who had entered through the patio, went straight to the silver chest. The comma should not be used when a clause is restrictive (i.e., essential to the meaning of the sentence): The burglar who had entered through the patio went straight to the silver chest; the other burglar searched for the wall safe.
- Sets off words or phrases in apposition to a noun or noun phrase: Plato, the famous Greek philosopher, was a student of Socrates. The comma should not be used if such words or phrases precede the noun: The Greek philosopher Plato was a student of Socrates.
- Sets off transitional words and short expressions that require a pause in reading or speaking: Unfortunately, my friend was not well traveled. Did you, after all, find what you were looking for? I live with my family, of course.
- Sets off words used to introduce a sentence: No, I haven't been to Paris. Well, what do you think we should do now?
- Sets off a subordinate clause or a long phrase that precedes a principal clause: By the time we found the restaurant, we were starved. Of all the illustrations in the book, the most striking are those of the tapestries.
- Sets off short quotations and sayings: The candidate said, “Actions speak louder than words.” “Talking of axes,” said the Duchess, “chop off her head.”—Lewis Carroll
- Indicates omission of a word or words: To err is human; to forgive, divine.
- Sets off the year from the month in full dates: Nicholas II of Russia was shot on July 16, 1918. But note that when only the month and the year are used, no comma appears: Nicholas II of Russia was shot in July 1918.
- Sets off city and state in geographic names: Atlanta, Georgia, is the transportation center of the South. 34 Beach Drive, Bedford, VA 24523.
- Separates series of four or more figures into thousands, millions, etc.: 67,000; 200,000.
- Sets off words used in direct address: “I tell you, folks, all politics is applesauce.”—Will Rogers. Thank you for your expert assistance, Dolores.
- Separates a tag question from the rest of a sentence: You forgot your keys again, didn't you?
- Sets off sentence elements that could be misunderstood if the comma were not used: Some time after, the actual date for the project was set.
- Follows the salutation in a personal letter and the complimentary close in a business or personal letter: Dear Jessica, Sincerely yours, Fred.
- Sets off titles and degrees from surnames and from the rest of a sentence: Walter T. Prescott, Jr.; Gregory A. Rossi, S.J.; Susan P. Green, M.D., presented the case.
See also: The Comma: A Major Player.