10 Functions of the Comma

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A comma is a versatile punctuation mark, serving ten 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 remainder of the sentence.

4. Separate an optional parenthetical element from the remainder 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.)

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 remainder 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 Avenue, Washington, DC”) (and dates (“January 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.”)

3 Appositive Errors

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Confusion often arises as to how or whether to punctuate appositive phrases, which are descriptions that identify someone or something named in the same sentence. Here are three statements with punctuation problems that illustrate the peril of improper punctuation, with explanations and suggested revisions.

1. “The fog arrived unannounced — ‘on little cat feet,’ as the American poet, Carl Sandburg, put it.”
Framing Carl Sandburg’s name with commas implies that he is the American poet — the only one. The accurate revision is “The fog arrived unannounced — ‘on little cat feet,’ as the American poet Carl Sandburg put it.” (In this case, the, preceding the epithet, is optional — and American is included only because the source sentence is from a book published in the United Kingdom, where Sandburg’s name is not as well known as it is in the United States.)

2. “But he had another particular passion, and that was water, ‘especially dramatically moving water,’ writes his biographer Robert Jones.”
It’s quite possible for a famous personage to have more than one biographer, but in this context, only one, the source of the quote, is referred to, so his name should be set off in apposition to the epithet biographer: “But he had another particular passion, and that was water, ‘especially dramatically moving water,’ writes his biographer, Robert Jones.” (In other contexts, “writes biographer Robert Jones” — which from its lack of commas denotes that more than one biographer exists — might be correct.)

3. “Here’s what the CEO of World Wide Widgets John Smith said to his employees in a blog post.”
The identification of the subject of this sentence is incorrectly ordered. Four solutions present themselves: “Here’s what World Wide Widgets CEO John Smith said to his employees in a blog post” is journalistic style, in which the simple affiliation-title-name syntax erases the need for punctuation, but formal writing favors a more relaxed arrangement.

“Here’s what John Smith, World Wide Widgets’s CEO, said to his employees in a blog post” helpfully sets the subject’s affiliation and title off from his name, but it’s better yet to reverse the order to title, then affiliation; either that combination or the name can come first (“Here’s what the CEO of World Wide Widgets, John Smith, said to his employees in a blog post” or “Here’s what John Smith, the CEO of World Wide Widgets, said to his employees in a blog post”; in the latter example, the is optional).