Commas are one of the most used (and misused) punctuation marks. Instead of falling into the trap of thinking you should insert a comma at each natural pause in a sentence, use this as a quick reference guide.
Independent Clauses
Use a comma before a coordinating conjunction (and, but, for, or, nor, yet, so) that joins two independent clauses in a compound sentence. (An exception can be made if the clauses are very short.) Just be sure the conjunction is joining two clauses. Do not place a comma between the elements of a compound verb.
Sunlight glittered on the lake, and birds chirped in the trees.
BUT
Sunlight glittered on the lake and forced me to wear sunglasses.
Coordinate Adjectives
Use a comma between coordinate adjectives (two or more adjectives each separately modifying the same noun). You can tell adjectives are coordinate if they can be joined by “and” and placed in a different order. Do not place a comma between cumulative adjectives (adjectives that have to be placed in a certain order to make sense).
A meticulous, thorough editor is essential to producing professional content.
BUT
She wore the long red dress.
Series
Use a comma to separate three or more words or phrases in a series. If you are using The Chicago Manual of Style, place a comma before the last item in the series. If you are following The AP Stylebook, skip it, unless the meaning would be ambiguous without it.
Tom, Bill, and Suzy greeted me warmly. (Chicago style)
Tom, Bill and Suzy greeted my warmly. (AP style)
Dialogue Tags
Use a comma to separate speech from dialogue tags. However, if the quotation needs a question mark or exclamation point, do not use a comma. Also, skip the comma if what is inside the quotation marks flows grammatically with the sentence.
She said, “Don’t touch that stove!”
BUT
“Don’t touch that stove!” she said.
She said the stove was “very hot.”
Locations and Dates
Use a comma to set off geographical locations and dates (when the month, day, and year are used).
She was born January 15, 1972, in New Berlin, Wisconsin, on a cold, snowy day.
Introductory Words and Phrases
Use a comma to set off a word or phrase that introduces a sentence, particularly an interjection, an introductory participial phrase, a transitional word, or a dependent clause that precedes the main clause (but not one that follows it).
Oh, I hope I am not late.
Running as fast as she could, she burst into the room.
Indeed, she was late.
Because my alarm clock did not go off, I was late to class.
BUT
I was late to class because my alarm clock did not go off.
Nonrestrictive Information
Use a comma to set off a word or phrase that is nonrestrictive; that is, it can be left out of the sentence without changing the meaning, particularly a parenthetical element or an appositive (that renames a noun).
The dog, who hates cats, spent his day barking up a tree. (Extra information)
She spoke to her mom, Clarice, on the phone. (She only has one mom. The name is an appositive that gives additional information not essential to the meaning of the sentence.)
BUT
The dog who lives in that yard spent his day barking up a tree. (Restrictive and essential to identifying which dog)
She spoke to her sister Julie on the phone. (She has more than one sister, so the name is restrictive and essential to clarifying which sister she called.)
Clarity
Use a comma to separate two words that might be unclear when read together.
What the problem is, is not clear.
Direct Address
Use a comma to set off a direct address.
I will be home late, Dad.
Many questions about commas can be cleared up by deciding whether the word or phrase is restrictive or nonrestrictive. When in doubt, consult a style guide (or your friendly neighborhood copy editor). The Chicago Manual of Style has helpful sections on punctuation and addresses a few rarer issues I did not discuss here. As always, the goal is ease of reading, so keep the reader in mind.