The comma functions as a tool to indicate to readers a certain separation of words, phrases, or ideas in order to prevent misreading the writer's intended meaning. When a sentence is spoken aloud, a comma often represents a pause, which in verbal conversation functions to clarify meaning. The comma is used according to specific rules that relate to grammatical structures within the sentence. Consistency in the use of commas allows the reader to be assured of proper interpretation of the writer's intentions.
Use of the comma can be categorized into ten rules. In English there are always exceptions to every rule, but in general, if the situation does not meet the requirements of one of these rules, a comma is most likely not necessary.
1. Use a comma before a coordinating conjunction joining independent clauses.
Carol spotted her favorite butterfly, but she had forgotten to bring her camera.
Gloria took a trip to Oroville, and her friend Pat joined her there.
I work hard and I play hard.
2. Use a comma to separate items in a series.
Their new kitten was frisky, playful, coy, and mischievous.
The apartment, the car, and the books were more than I could afford.
Their new kitten was frisky and playful.
The apartment and the car were more than I could afford.
Their new kitten was frisky and playful and coy and mischievous.
The apartment and the car and the books were more than I could afford.
In the following example, it could be unclear who is included in each section:
We will have separate sections for friends of the bride and groom, relatives of the bride and groom and the bridal party and their escorts
The next two examples show that, depending on how the writer intended to separate the groups of people, the placement of a comma before the and of the last item clears up the confusion:
We will have separate sections for friends of the bride and groom, relatives of the bride and groom, and the bridal party and their escorts. (In this situation, the bridal party and their escorts sit together in their own section separately from relatives of the bride and groom.
We will have separate sections for friends of the bride and groom, relatives of the bride and groom and the bridal party, and their escorts. (In this situation, the relatives of the bride, groom, and bridal party all sit together while the escorts that came with them sit separately in their own section.
Without the comma in the following sentence, yoga could be a topic of the scary stories:
You may choose your favorite activity while at camp, including hiking and boating, morning calisthenics, crafts and woodworking, sharing scary stories of ghosts and yoga.
Placing a comma before and clarifies that yoga is a separate activity of its own:
You may choose your favorite activity while at camp, including hiking and boating, morning calisthenics, crafts and woodworking, sharing scary stories of ghosts, and yoga.
In the following example, if no comma is required before the last item in the series, it is not clear whether there are four divisions of the estate or three:
The benefactor will divide his estate equally among Fran and Charlie, Tyler and Danielle, Connor and Yvette.
The addition of a comma prior to and clarifies that the estate will be divided four ways:
The benefactor will divide his estate equally among Fran and Charlie, Tyler and Danielle, Connor, and Yvette.
3. Use a comma after certain introductory words or word groups.
Introductory adverbial (dependent) clause:
After the war in Troy was over, Ulysses started home to Ithaca.
(Note that if the dependent clause comes after the independent clause, there is no need for a comma: Ulysses started home to Ithaca after the war in Troy was over.
Introductory prepositional phrase:
Near the house at the end of the road, you will find the little wagon.
Ironically, Danielle has no realization that she is so highly admired.
Introductory adjective phrase:
Pretty as a picture, Betsy sauntered down the steps.
Introductory participial phrase:
Having just finished an ice cream sundae, he had no desire to taste the pie.
Introductory infinitive phrase:
To save money, I walk to work and bring a bag lunch from home.
Introductory transitional expressions:
Therefore, we decided not to go.
After dinner I always take a two-mile walk.
By evening the snow was thick.
4. Use commas to set off transitional and parenthetical expressions, absolute phrases, and contrasted elements.
Steph's car, as far as I know, is not running at all.
Their apartment, however, was quite near the subway.
Their walking skills perfected, they made good use of the tree-lined sidewalks.
Unlike most students, Tyler was always ahead on his assignments.
5. Use commas to set off nonrestrictive elements. Do not use commas to set off restrictive elements.
Food which is high in calories often tastes very good.
Ice cream, which is not included in my diet, is a food I try to avoid.
6. Use a comma between coordinate adjectives not joined by and. Do not use a comma between cumulative adjectives.
It was a dull, dark, and depressing day.
With coordinate adjectives, changing the order or adding and does not change the meaning:
It was a dark, depressing, and dull day.
It was a dull and dark and depressing day.
She ordered a piece of rich chocolate layer cake.
With cumulative adjectives, it does not work to rearrange them or join them with and:
Not correct: She ordered a piece of rich and chocolate and layer cake.
Not correct: She ordered a piece of layer chocolate rich cake.
7. Use commas to set off nouns of direct address, the words yes and no, interrogative tags, and minor interjections.
Well, it's certainly good to see you here, Conrad.
Yes, I remember you as an exceptionally fine soccer player.
You're in medical school now, aren't you?
8. Use commas to set off direct quotations.
"You cannot consider yourself an environmentalist," the author clearly stated, "until you've visited Humboldt Redwoods State Park in California."
9. Use commas to separate certain items in dates, addresses, titles, and numbers
He was born on February 16, 1977, in Morgantown, West Virginia.
Tyler set up camp at an altitude of 14,746 feet.
Dr. Yvette Shannon, D.O., diagnosed the rare disease and saved the patient's life.
10. Use a comma to indicate a pause when it is necessary to prevent confusion in the meaning of the sentence.
What we were hoping would happen, happened.
The children who can, swim laps every morning.
Reminder: In general, avoid using commas unless one of the above rules indicates that you should.
between a subject and verb.
between an adjective and noun.
between an adverb and adjective.
before the first or after the last item in a series.
after a coordinating conjunction.
after such as or like.
directly before or after a question mark or exclamation point.
between compound elements that are not independent clauses.