A comma is a versatile punctuation mark, serving 10 basic functions. Here’s an enumeration, with examples.
1. Separate the elements in a series: “Groucho, Harpo, and Chico developed the philosophy called Marxism.”
Many periodicals and websites, and most colloquially written books, omit the serial, or final, comma, but it is all but mandatory in formal writing and is recommended in all usage. As language maven Bryan Garner observes, “Omitting the serial comma may cause ambiguities, whereas including it never will.”
2. Separate coordinated independent clauses: “I like the Marx Brothers, but she thinks they’re too silly.” (An independent clause is one that can stand on its own as a sentence but is linked with another by a conjunction and/or a punctuation mark.)
Exceptions include sentences with closely linked clauses (“Go to the window and see who’s there”) and those with a compound predicate (“The Marx Brothers are known for their puns and their sight gags”).
3. Separate an introductory word (“Naturally, I agree with you”), phrase (“Last summer, I went on a long vacation”), or subordinate clause (“If you’re too busy now, wait until later”) from the rest of the sentence.
4. Separate an optional parenthetical element from the rest of the sentence. “We have, in a manner of speaking, won despite our loss.” (The phrase “in a manner of speaking” could also be set off by em dashes or parentheses, depending on whether the writer wishes to emphasize the interruption of the statement “We have won despite our loss” or wants to diminish it as an aside.)
5. Separate coordinate adjectives from each other: “I could really use a tall, cool drink right now.” (Do not separate noncoordinate adjectives with a comma; this post explains the difference between these two types of adjectives.)
6. Separate an attribution from a direct quotation: “She said, ‘Neither choice is very appealing'”; “‘That’s not my problem,’ he replied.” (A colon may be precede a formal pronouncement or an attribution that forms a complete thought, as in, “He had this to say: ‘Her point is irrelevant.” Omit punctuation when the attribution is implied, as in “Your response ‘Her point is irrelevant’ is evasive.”)
7. Separate a participial phrase or one lacking a verb from the rest of the sentence: “Having said that, I still have my doubts”; “The deed done, we retreated to our hideout.”
8. Separate a salutation from a letter (“Dear friends,”) or a complimentary close from a signature in a letter (“Sincerely,”). A colon should be used in place of a comma in a formal salutation.
9. Separate elements when setting off a term for a larger geopolitical entity from that for a smaller one located within it (“Santa Barbara, California, is located on the coast”) and for elements of street addresses (“1600 Pennsylvania Ave., Washington, DC”) and dates (“Jan. 1, 2013”).
10. Separate groups of three digits in numbers: “Let me tell you how to make your first 100,000,000 dollars.” (Because large numbers are difficult to scan, it’s usually better to use one of the following forms: “100 million dollars,” “one hundred million dollars,” or “$100 million.”)
A version of this article originally appeared on Daily Writing Tips.