When to Use a Comma

1. Use a comma to separate independent clauses (complete sentences) when they are joined by the coordinating conjunctions and, but, or, nor, for, so, yet.

  • The students studied all night for the test, but no one made a passing grade.

  • Every computer in the lab was in use, so I walked all the way to the library in the cold.

Note: Beware of comma splices, an error which occurs when two complete sentences are joined with only a comma and no coordinating conjunction.

  • The lab assistant checked her results, she didn't want to make a mistake. --incorrect

  • The lever should be switched to full power, sealant should then be applied. --incorrect

2. Use a comma after long introductory clauses, phrases, or words that come before the main clause.

  • If every student in our club helped, this fundraiser would be our most successful yet.

  • Because he forgot to set his alarm, he was late for class.

  • At the racetrack, Henry won enough money to buy everyone dinner.

3. Use a pair of commas in the middle of a sentence to set off phrases, clauses, and words that are not essential to the meaning of a sentence.

  • My teacher, Katherine Grey, told me I should study more.

  • Apples that are green are usually Granny Smith apples. (essential clause)

  • Drivers who have been convicted of drunken driving should lose their licenses. (essential clause)

  • Apples, which are my favorite fruit, are a convenient sack lunch item. (non-essential clause)

  • The students protesting the assignments refused to come to class. (essential clause)

  • The students, protesting the assignments, refused to come to class. (non-essential clause)

4. Use a comma to separate three or more words written in a series.

  • I earned an A in calculus, physics, English, and history last semester.

  • I sent the memo to Accounting, Marketing, and Research and Development.

  • Diners have a choice of broccoli, green beans, peas, or carrots.

5. Use a comma to separate two or more coordinate adjectives that describe the same noun.

Coordinate adjectives usually:

Can be written in reverse order

Make sense with an AND between them

  • She was a greedy, stubborn child. (coordinate adjectives)

  • The cracked bathroom mirror reflected his face. (no coordinate adjectives)

6. Use a comma to set off geographical names, items in dates (except month and day), addresses, and titles in names.

  • Birmingham, Alabama, gets its name from Birmingham, England.

  • This letter concerns the memo I sent on July 23, 2001, to every member of the staff.

  • Thousands swarmed the wall in November, 1989, to demolish it.

  • N. Katherine Hales, Ph.D., will be the lecturer today.

7. Use commas to set off direct quotations and dialogue.

  • The manager replied, ?I think we all want to complete this project.?

8. Use commas anywhere in the sentence to prevent possible confusion or misreading.

  • To Jonathan, Eliot had been a sort of mentor.

  • When he cooks, John often makes a mess.

Created by April Whitman, University of Oklahoma