Correct comma usage is an art, not a science. Writers will sometimes disagree on when to use a comma or omit one; to help you as you write, here are a few general rules for using commas. These examples and the rules they’re based upon are drawn from the 16th edition of the Chicago Manual of Style, one of the core texts on proper language usage and formatting.
Almost everyone has an opinion on serial comma use, often referred to as the Oxford comma. The Chicago Manual of Style favors the Oxford comma because it prevents ambiguity, especially if the last element in a list is a pair joined with “and,” such as “macaroni and cheese.”
They are going to the store with my sister, brother, and cousin.
If there is an ampersand in the sentence, there is a comma before the ampersand, such as in a citation (in APA format):
Colonies of ants have long held a distinct position in ecclesiastical art (Reuben, Johnson, & Carlyle).
Omitting a serial comma can create ambiguity:
I had lunch with my parents, the Queen and the Prime Minister of England.
In this sentence, the reader could easily misread this to mean your parents are the Queen and the Prime Minister of England. Adding a comma after “Queen” makes each of the items clearly distinct.
Clauses and Phrases: Restrictive and Non-Restrictive
The restrictive clause “provides information…essential to the meaning of the sentence” (6.22). In other words if the clause is omitted, the meaning of the sentence changes. These clauses are not set off by commas:
The man who was coming to dinner was Sidney Poitier.
In this example, “the man” could be anyone. It’s the information that specifies it’s not just any man, but the one who is coming to dinner, that lets us know who the man is.
Non-restrictive clause and phrases don’t change the sentence meaning if they are left out.
Here we know who the man is—John—and so the clause simply tells you more about John. If the clause is left out, the sentence still has the same meaning.
Independent and Dependent Clauses
If a clause can stand on its own and doesn’t rely on the rest of the sentence for meaning, then it’s an independent clause. Set these clauses off with a comma.
The camel stood outside the gates, but I was on the inside of the city.
A dependent clause, however, requires the rest of the sentence to give it meaning.
We will not give the child dessert if she does not finish dinner.
John left with Ted, and Mary left soon after.
In the example above, if you don’t use a comma after “Ted,” the reader will momentarily misread it as “John left with Ted and Mary . . .” and then have to reread the sentence to understand what the writer actually intended.
Here’s a general rule for using the comma correctly: Focus on how the reader will interpret your meaning, and eliminate any possible ambiguity. With a little bit of practice, you’ll begin using commas like a pro!