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.”)

7 Cases for Inserting or Omitting Commas

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Here are discussions of seven types of situations in which the presence or absence of a comma depends on various factors.

1. Word Function

Whether a comma follows a word sometimes depends on the function of the word. For example, when now is employed at the head of a sentence to refer to the present time, there’s no reason to separate it from the rest of the statement: “Now you know.”

But when now serves as an interjection to mark a transition or attract someone’s attention, it should be set off: “Now, have you had dinner?” (That same series of words could be used in a temporal sense, though: “Now have you had dinner?” suggests that the writer is impatient with the person the question is directed to.)

2. Before Because

A sentence such as “I didn’t want to go because I hadn’t enjoyed myself last time” implies that the writer is explaining that the lack of enjoyment isn’t a factor in reluctance to attend an event; the reason for the reluctance will presumably follow.

But if the meaning is opposite — if the lack of enjoyment is the reason for the reluctance to attend — a comma should precede because to signal that what follows the comma is a dependent clause: “I didn’t want to go, because I hadn’t enjoyed myself last time” Alternatively, the dependent clause can begin the sentence: “Because I hadn’t enjoyed myself last time, I didn’t want to go.”

3. Apposition

An appositive is a word or phrase equivalent in meaning to an adjacent word or phrase, as in “She wrote to her brother, John”; “her brother” and “John” mean the same thing, so they are appositives, and the comma is necessary to set it off. However, if the woman has more than one brother, write “She wrote to her brother John.”

Similarly, in “I met the writer, Jane Doe,” the comma is correct only if the writer has been alluded to before without mention of her name. Otherwise, the comma between the appositives suggests that only one writer exists. (And that puts me out of a job.) Even if writer is modified, the meaning differs: “I met the mystery writer, Jane Doe” suggests a previous reference to two or more writers, only one of whom writes mysteries, whereas “I met the mystery writer Jane Doe” simply specifies the genre in which Jane Doe writes.

4. Relative Clauses

Commas may or may not be necessary, depending on whether each statement in an otherwise identical pair of sentences uses the word that or which: In “The house that Jack built is falling apart,” the phrase “that Jack built” is essential to the sentence, which specifies a particular house. In “The house, which Jack built, is falling apart,” the emphasis is on what is happening to the house, and the identity of the builder is a parenthetical, so the optional information should be bracketed by commas.

“The house which Jack built is falling apart,” without commas, is also correct; it is identical in meaning to “The house that Jack built is falling apart.” However, the convention in American English is to avoid using which in this sense to prevent confusion with the meaning of the sentence with the parenthetical phrase.

5. Short Introductory Phrases

Many people choose to omit a comma after introductory phrases of just a few words, as in “During the summer I like to travel.” However, such omission is arbitrary when such sentences are compared to those with longer introductory phrases — and wrong in the case of transitional tags like finally, furthermore, and unfortunately — and for the sake of consistency, a comma should follow any introductory word or phrase.

6. Short Independent Clauses

In brief sentences such as “I will sort and you can staple” that consist of two independent clauses (complete thoughts that could stand on their own as distinct sentences), writers often choose to omit the otherwise obligatory comma before the conjunction.

But just as in the case of short introductory phrases, there is the problem of where to draw the line. Does one establish a rule about how many words each clause must contain to dictate whether a comma is employed, or does one judge each sentence on its own? Let simplicity be your guide: Always include a comma.

7. Coordinate and Noncoordinate Adjectives 

When two or more adjectives sequentially modify a noun, depending on their relationship, they may or may not be separated by commas. To test whether to insert or omit commas, replace them with and. For example, “She was wearing a bright, cheerful expression” can also be written “She was wearing a bright and cheerful expression.” (The adjectives can be reversed in either case, too.)

However, “She was wearing a dark green blouse” cannot be rendered “She was wearing a dark and green blouse,” because dark and green describe the blouse in combination, whereas bright and cheerful separately describe the expression. Also, in this case, the adjectives cannot be reversed: “She was wearing a green dark blouse” is illogical because dark modifies green, not blouse. Therefore, no comma should separate the two terms.

5 Cases of Excessive Commas

The rules about commas can seem so complicated — and contradictory — that writers can (almost) be forgiven for tossing in an extra one or two. Here are several examples of overly generous deployment of commas.

1. “If a killer asteroid was, indeed, incoming, a spacecraft could, in theory, be launched to nudge the asteroid out of Earth’s way, changing its speed and the point of intersection.”
This thirty-word sentence is littered with six commas — one for every five words — five of them appearing before the halfway point. By simply bending the rule about bracketing interjections with commas — a rule that advocates of open punctuation flout routinely anyway — the number is reduced by two, rendering the sentence more free flowing: “If a killer asteroid was indeed incoming, a spacecraft could, in theory, be launched to nudge the asteroid out of Earth’s way, changing its speed and the point of intersection.”

One more comma can be eliminated by relocating the parenthetical phrase “in theory” to an earlier position in the sentence, so that the comma after incoming does double duty: “If a killer asteroid was indeed incoming, in theory, a spacecraft could be launched to nudge the asteroid out of Earth’s way, changing its speed and the point of intersection.”

2. “The metaphor, ‘The world is a machine,’ began to replace the metaphor, ‘The world is a living organism.’”
In this sentence, the comma preceding each instance of metaphor implies that that metaphor is the only one — not just in the sentence, but anywhere. (But two metaphors are expressed here, and innumerable others exist.) Metaphor, appearing in apposition to the two brief quotations, should not be set off from them: “The metaphor ‘The world is a machine’ began to replace the metaphor ‘The world is a living organism.’”

3. “The event is part of a catchy, public health message about the importance of emergency preparedness.”
Catchy and “public health” are not coordinate adjectives. The point is not that the message is catchy and public health; it’s that the public health message is catchy. Therefore, no comma is necessary: “The event is part of a catchy public health message about the importance of emergency preparedness.”

If, by contrast, the sentence read, for example, “The event is part of a catchy, quirky message about the importance of emergency preparedness,” note that because catchy and quirky are parallel — they are coordinate adjectives — a comma should separate them.

4. “The report was completed in December, 2012.”
A comma is necessary between a month and a year only if a date is specified (“The report was completed on December 1, 2012”): “The report was completed in December 2012.” (The same rule applies when the name of a season appears in place of the name of a month: “The report was completed in fall 2012.”)

5. “Jones traveled by boxcar from California to New York with fellow fledgling artist, John Smith, sketching the American landscape along the way.”
Commas are necessary with this type of apposition only if the epithet is preceded by an article (“Jones traveled by boxcar from California to New York with a fellow fledgling artist, John Smith, sketching the American landscape along the way”): “Jones traveled by boxcar from California to New York with fellow fledgling artist John Smith sketching the American landscape along the way.” Unfortunately, this type of error has gone viral — its ubiquity is mistaken for propriety — and is seemingly ineradicable.

5 Cases for Requiring a Comma Before a Sentence Tag

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A sentence tag is a word or phrase added to the beginning or end of a statement for emphasis or to provide more information. For the following sentences, I discuss the necessity of preceding end-of-sentence tags with a comma.

1. “I shouldn’t have been surprised really.”

Without a comma separating really from the rest of the sentence, the implication is that really is an adverb modifying how the writer should not have been surprised (really, as in factually, and the opposite of allegedly). However, its function is merely to emphasize the point: “I shouldn’t have been surprised, really.”

2. “I was in the other room at the time actually.”

This sentence indicates that the writer was in the other room in an actual manner, rather than figuratively, but that’s not the literal meaning. The writer has been challenged about his or her location when an incident occurred, and the intent, again, is to emphasize. A comma is required before actually to signal this distinction: “I was in the other room at the time, actually.”

The idea could also be conveyed with actually inserted elsewhere in the sentence (in descending order of elegance): “Actually, I was in the other room at the time” or “I was, actually, in the other room at the time” or “I was in the other room, actually, at the time.” (Note that not all adverbial tags are so flexible about location; try these variations in the first example, and you’ll see that really seems to feel right only as a concluding tag.)

3. “We did it all right.”

This sentence implies that the writer is evaluating a merely competent performance. With a comma inserted before “all right,” the implication is of emphasis on the fact of the accomplishment: “We did it, all right.”

4. “They offered a free pass to boot.”

Without a comma preceding “to boot” (which means “as a bonus”), the phrase appears to describe an action that is, thanks to the pass, complimentary. The comma signals that “to boot” is an appendage that idiomatically offers additional information: “They offered a free pass, to boot.”

5. “Geology has an impact on biology and vice versa.”

As written, this sentence seems to equate biology and vice versa as two things geology has an impact on. But “vice versa,” meaning “the opposite,” applies to the entire sentence preceding it, so it must be set off from the sentence: “Geology has an impact on biology, and vice versa.”