Commas are used for a variety of functions. Here's a list of the most common uses.
- Use a comma before a coordinating conjunction like "and," "but," or "or" that joins independent clauses: Nearly everyone had arrived, but the guest of honor was late.
- Use a comma after an introductory phrase or clause. These introductory phrases often establish setting, either time or location: Near a small stream, the park rangers discovered the abandoned shack. In 1988, the shack had been abandoned.
- Use commas between all items in a list or series: Bubbles of air, leaves, ferns, and bits of wood are found trapped in sap.
- Use a comma between two or more adjectives that each modify a noun separately, called coordinate adjectives: Robert is a handsome, brave, and generous man.
- Do not use commas between adjectives that modify a noun when placed together, called cumulative adjectives: Three large grey shapes moved towards us. NOT Three, large, grey shapes moved towards us.
- Use commas to set of phrases if they could be eliminated from the sentence without changing its meaning: The helicopter, with its spotlight, circled above.
- Use commas to set off transitional phrases: As a matter of fact, American football dates back to the Middle Ages.
- Use commas around parenthetical phrases: Evolution, as far as we know, doesn't work this way.
- Use commas to set off other elements of a sentence:
- Names when directly addressing a person: Forgive us, Mrs. Smith, for breaking the dish.
- The words "yes" and "no": Yes, I'll be there at 6:00.
- Tag questions: The book was better than the movie, wasn't it?
- Interjections: Well, it's difficult to say.
- Use commas in numbers, addresses, dates, and titles:
- Numbers: 20,000
- Addresses: 123 Main St., Marquette, MI 49855
- Dates: On July 4th, 1776, the Declaration was signed.
- Titles: Susan Hawkins, M.D., will present her theory at the meeting.
Compiled from Hacker, Diana. Rules for Writers. 5th ed. Boston: Bedford/St. Martin’s, 2004.