A comma signals a pause in a sentence, either to introduce or connect ideas, separate items in a series, or set off a quotation or interrupted thought.
You should use commas in five basic situations in a sentence:
- After an introductory phrase of more than three or four words. The comma signals the end of the introductory phrase and the start of the main part or idea of the sentence.
Example: After a prenuptial ceremony in a sweat lodge, the couple gathered with friends on a mountain top for the wedding.
Tip: The main part of the sentence should be able to stand alone as a complete thought.
- At the beginning and end of a phrase that interrupts the main thought. These commas show that the information between them is "extra."
Example: The graduate student's paper, in my opinion, was brilliantly written and should be submitted to a journal.
Tip: Put commas around a phrase if you can lift it out of the sentence without changing its meaning.
- To separate items in a series.
Example: Jane walked into her teenager's bedroom and found a half-eaten tuna sandwich, a collection of moldy socks, an empty cigarette carton, and a pile of unopened textbooks.
Example: Sean found a cup of stone-cold coffee, an empty yogurt carton, an open bottle of aspirin, and 14 stacks of unread reports when he walked into his mother's office.
Tip: In journalism, the comma often is omitted after the next-to-last item in a series. In academic writing, you usually include the comma. However, any time that you omit this comma (sometimes called an Oxford comma), you may cause unnecessary confusion. For example, "Her picture of her family around the fire pit included her grandfather, her grandmother, her stepmother, her sister-in-law, her brother holding his daughter and the family dog." So, is her brother holding a child AND the dog or is the daughter actually a fur-baby? This sentence would be much clearer with the Oxford comma.
- Before a connecting word (conjunction) to link two complete sentences together.
Example: I could call my boss and grovel for the day off, or I could assert my right to take a mental-health day.
Tip: A comma does not link two complete sentences together. It must have a connecting word.
- To introduce a quotation in a sentence.
Example: Joanna repeated the medium's message, "You either must make peace with your brother, or you'll be doomed to repeat the same relationship in the next life."
Example: "And I tell you, my fellow Americans," the politician exhorted, "the middle class will not bear the brunt of new taxes."
Other Common Purposes
To separate the day from the year in a date
Example: November 22, 2001
Tip: When more of the sentence follows the date, you also need a comma after the year.
Example: I was born on Feb. 18, 1954, in Olean, N.Y.
To separate a city from a state
Example:Lackawanna, New York
Tip: When more of the sentence follows after city and state, you also need a comma after the state.
Example: Empire State College's administrative offices are located in Saratoga Springs, N.Y., on the same street as the famous race track.
To separate a person's name and title
Example: Madeline Albright, Secretary of State
Tip: When more of the sentence follows after the name and title, you also need a comma after the title.
Example: Dan Glickman, the Agriculture Secretary, recalled 25 million ears of corn in August.
Example: The Florida Marlins' World Series triumph was a dream come true for Livan Hernandez, Most Valuable Player.
After an introduction to a personal letter
Examples: Dear Dr. Belasen,
In long numbers, to separate numbers into units of three
Tip: It's optional to use a comma in a four-digit number. MLA style is to use a comma in four-digit numbers except in page numbers, line numbers, addresses, and years, unless the year has more than four digits, such as 10,000 BCE.
For more information about comma use, consult a handbook such as "A Writer's Reference (Ninth Edition, 2018)" by Diana Hacker and Nancy Sommers or this helpful resource.