A comma is used to
- group words that belong together
- separate words that do not belong not belong together
- assist readers in making sense of sentences
- follow specific established conventions.
- to set off introductory phrases and clauses:
After the rain, the grass grew taller.
To see the microbe, we took turns looking into the microscope.
- to set off parenthetical elements that provide transitions, express afterthoughts, or offer supplemental information:
The best Tim Burton movie is, in my opinion, Pee-Wee's Big Adventure.
before coordinating conjunctions (and, but, or, nor, for, yet, so) linking two independent clauses
My sister wants a car for graduation, and my parents intend to buy her one.
BUT NOT: My sister wants a car for graduation, and a trip to Europe. Note: and does not link two independent clauses.
- to separate items in a series (three or more things):
Let's order a pizza with pepperoni, green peppers, and mushrooms.
Let's drive to Sioux Falls, eat at Granite City, and watch a movie on Saturday night.
Nonrestrictive elements provide information that is not essential to the meaning of the sentence:
Tom Cruise, a well-paid actor, lives in a luxurious mansion in Hollywood.
Note: restrictive elements, which are crucial to the meaning of the sentence, are not set off by commas:
Actors who are well-paid tend to live in luxurious mansions in Hollywood. (The predicate does not apply to all actors, but only those who are well paid.)
Note: The relative pronoun "that" always indicates a a restrictive clause and so should NOT be preceded by a comma.
Actors that are well-paid tend to live in luxurious mansions in Hollywood.
DO NOT use commas, unless sentence elements intrude that require commas, in the following cases:
- between a subject and verb
- between a verb and its object
- between a preposition and its object
- between compound words or phrases joined by a coordinating conjunction (and)
- after "such as" and "like"