Contrary to popular belief, commas don't just signify pauses in a sentence.
In fact, precise rules govern when to use this punctuation mark. When followed, they lay the groundwork for clear written communication.
We've compiled a list of all of the times when you need the mighty comma.
1. Use a comma before any coordinating conjunction (and, but, for, or, nor, so, yet) that links two independent clauses.
Example: "I went running, and I saw a duck."
You may need to learn a few grammatical terms to understand this one.
An independent clause is a unit of grammatical organization that includes both a subject and verb and can stand on its own as a sentence. In the previous example, "I went running" and "I saw a duck" are both independent clauses, and "and" is the coordinating conjunction that connects them. Consequently, we insert a comma.
If we were to eliminate the second "I" from that example, the second clause would lack a subject, making it not a clause at all. In that case, it would no longer need a comma: "I went running and saw a duck."
2. Use a comma after a dependent clause that starts a sentence.
Example: "When I went running, I saw a duck."
A dependent clause is a grammatical unit that contains both subject and verb but cannot stand on its own, like "When I went running ..."
Commas always follow these clauses at the start of a sentence. If a dependent clause ends the sentence, however, it no longer requires a comma. Only use a comma to separate a dependent clause at the end of a sentence for added emphasis, usually when negation occurs.
3. Use commas to offset appositives from the rest of the sentence.
Appositives act as synonyms for a juxtaposed word or phrase. For example, "While running, I saw a mallard, a kind of duck." "A kind of duck" is the appositive, which gives more information about "a mallard."
If the appositive occurs in the middle of the sentence, both sides of the phrase need a comma. As in, "A mallard, a kind of duck, attacked me."
Don't let the length of an appositive scare you. As long as the phrase somehow gives more information about its predecessor, you usually need a comma.
"A mallard, the kind of duck I saw when I went running, attacked me."
There's one exception to this rule. Don't offset a phrase that gives necessary information to the sentence. Usually, commas surround a non-essential clause or phrase. For example, "The duck that attacked me scared my friend" doesn't require any commas. Even though the phrase "that attacked me" describes "the duck," it provides essential information to the sentence. Otherwise, no one would know why the duck scared your friend. Clauses that begin with "that" are usually essential to the sentence and do not require commas.
4. Use commas to separate items in a series.
For example, "I saw a duck, a magician, and a liquor store when I went running."
That last comma, known as the serial comma, Oxford comma, or Harvard comma, causes serious controversy. Although many consider it unnecessary, others, including Business Insider, insist on its use to reduce ambiguity.
There's an Internet meme that demonstrates its necessity perfectly. The sentence, "We invited the strippers, JFK, and Stalin," means the speaker sent three separate invitations: one to some strippers, one to JFK, and one to Stalin. The version without the Oxford comma, however, takes on an entirely different meaning, potentially suggesting that only one invitation was sent — to two strippers named JFK and Stalin. Witness: "We invited the strippers, JFK and Stalin."
5. Use a comma after introductory adverbs.
"Finally, I went running."
"Unsurprisingly, I saw a duck when I went running."
Many adverbs end in "ly" and answer the question "how?" How did someone do something? How did something happen? Adverbs that don't end in "ly," such as "when" or "while," usually introduce a dependent clause, which rule number two in this post already covered.
Also insert a comma when "however" starts a sentence, too. Phrases like "on the other hand" and "furthermore" also fall into this category.
Starting a sentence with "however," however, is discouraged by many careful writers. A better method would be to use "however" within a sentence after the phrase you want to negate, as in the previous sentence.
6. Use a comma when attributing quotes.
The rule for where the comma goes, however, depends on where attribution comes.
If attribution comes before the quote, place the comma outside the quotations marks. The runner said, "I saw a duck."
If attribution comes after the quote, put the comma inside the quotation marks. "I saw a duck," said the runner.
7. Use a comma to separate each element in an address. Also use a comma after a city-state combination within a sentence.
"I work at 257 Park Ave. South, New York, N.Y. 10010."
"Cleveland, Ohio, is a great city."
8. Also use a comma to separate the elements in a full date (weekday, month and day, and year). Also separate a combination of those elements from the rest of the sentence with commas.
"March 15, 2013, was a strange day." Even if you add a weekday, keep the comma after "2013."
"Friday, March 15, 2013, was a strange day."
"Friday, March 15, was a strange day."
You don't need to add a comma when the sentence mentions only the month and year. "March 2013 was a strange month."
9. Use a comma when the first word of the sentence is freestanding "yes" or "no."
"Yes, I saw a duck when I went running."
"No, the duck didn't bite me."
10. Use a comma when directly addressing someone or something in a sentence.
My editor often asks, "Christina, is that article up yet?"
Another clever meme shows the problem with incorrect placement of this comma. "Stop clubbing baby seals" reads like an order to desist harming infant mammals of the seal variety. The version with a comma, however, instructs them to stop attending hip dance clubs. "Stop clubbing, baby seals."
11. Use a comma between two adjectives that modify the same noun.
For example: "I saw the big, mean duck when I went running."
Only coordinate adjectives require a comma between them. Two adjectives are coordinate if you can answer yes to both of these questions: 1. Does the sentence still make sense if you reverse the order of the words? 2. Does the sentence still make sense if you insert "and" between the words?
Since "I saw the mean, big duck " and "I saw the big and mean duck" both sound fine, you need the comma.
Sentences with non-coordinate adjectives, however, don't require a comma. For example, "I lay under the powerful summer sun." "Powerful" describes "summer sun" as a whole phrase. This often occurs with adjunct nouns, a phrase where a noun acts as an adjective describing another noun — like "chicken soup" or "dance club."
12. Use a comma to offset negation in a sentence.
For example: "I saw a duck, not a baby seal, when I went running."
In this case, you still need the comma if the negation occurs at the end of the sentence. "I saw a baby seal, not a duck."
Also use commas when any distinct shift occurs in the sentence or thought process. "The cloud looked like an animal, perhaps a baby seal."
13. Use commas before every sequence of three numbers when writing a number larger than 999. (Two exceptions are writing years and house numbers.)
For example, 10,000 or 1,304,687.
That concludes today's lesson. Check your understanding of some other common grammatical mistakes here.
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