The original purpose for the comma is so the reader knows where the pauses are located in a sentence. In fact, the word in Greek means “a piece cut off”.
There are six uses of the commas:
1) To separate items in a series (which is known as a “serial comma”). In a series of three or more words or phrases, a comma goes after every word or phrase:
Life, liberty, and the pursuit of happiness.
Carmen jumped out of the shower, grabbed a towel, and ran to the ringing phone.
2) To set off introductory materials.
After hitting the snooze button, Bobby went back to sleep.
In addition, Mary used nutmeg for her pie.
Sometimes a comma is used to set off additional phrases after a sentence:
I love to listen to jazz music, especially John Coltrane and Miles Davis.
A sudden breeze came through the open window, driving the stuffiness out of the room.
(NOTE: These phrases before and after the sentence are called dependent clauses, so the clauses cannot stand on their own.)
3) Before and after words that interrupt the flow of thought in a sentence.
The convict, said the judge, is mad.
Well, Stanley, this is another fine mess you’ve gotten us into.
The audience, which had been bored at first, quickly became interested in the play.
(NOTE: These inserted phrases are often called “parenthetic”. Also, the inserted phrase in the last sentence is called a nonrestrictive clause, in other words, it does not define or limit the subject, but add more information on the subject. In cases like this, the phrase is parenthetic. Phrases that define the subject are restrictive and so are not parenthetic.)
Let he who is without sin cast the first stone.
The dog that barked last night is amazingly quiet this morning.
4) Before two complete thoughts (or independent clauses) connected by such conjunction words as and, but, for, or, nor, so, yet:
The wedding was scheduled for four o’clock, but the bride changed her mind at two.
The hunters came running into the woods, and the bear took off.
(NOTE: Be careful of comma splices, which are usually caused by run-ons, or joining two independent clauses without a conjunction. These mistakes are usually common in new writers. While there are few exceptions, such as Easy come, easy go, it’s best to avoid comma splices.)
5) To set off a direct quotation from the rest of a sentence
Maude Lebowski asked, “What do you do for recreation?”
“Oh, the usual,” replied The Dude. “I bowl. Drive around. The occasional acid flashback.”
6) For certain everyday materials (such as addressing a person, dates, places, openings and closings of letters, numbers, etc.):
Donny, you are out of your element!
September 3, 2011
I live in Los Angeles, California
Truly yours, Amy
I traveled 23,000 miles to visit my father.