The comma is the most frequently used internal mark of punctuation. Of all the marks of punctuation, it has the widest variety of uses.
Using commas with dates, addresses, greetings, names, and large numbers
* Commas are used with full dates (month, day, and year) but omitted with partial dates (month and year):
1. Gas has been first used by the Germans on October 14, 1914, when they fired a prototype of modern tear gas from artillery near Pyres. – Paul Fussel
2. In June 1985 Beth Henley was working on her fifth play.
Exception: No comma is used to separate parts of a date that begins with the day.
The atomic bomb was first dropped on 6 August 1945.
* Commas are required between most of the elements in place names and addresses:
1. Miami, Dade County, Florida
2. Writing Lab, University of California, Riverside
Exception: # Do not use comma to separate street number from the name of the street:
15 Amsterdam Avenue
# Do not use comma to separate a state from zip code:
5625 Waverly Avenue, La Jolla, California 92037
* In complete sentence, a comma must follow the last element of place name, addresses, or dates:
1. He shot himself twice, once in the chest and then in the head, in a police station in Washington, D.C., with the cops looking on. – Red Smith
2. July 4, 1776, was the day the Declaration of Independence was signed.
* Commas are used to set off the names of someone directly addressed in the sentence:
A few years ago, Mr. Taplow, I spoke to you about the possibility of a summer job.
* Commas are used after the greeting in a friendly or informal letter, and after the closing of the letter of any kind:
* Commas are used to set off titles or degrees after a person's name:
Barbara Kane, M.D., delivered the commencement address.
Exception: But Jr., Sr. may be written without commas:
Sammy Davis Jr. started his singing career at age four.
* The comma is used after the last part of a proper name when the last part comes first:
Lunt, George D.
* Commas are used to mark groups of three digits in large numbers, counting from the right:
Antarctica is 5,400,000 square miles of ice-covered land.
Using commas with conjunctions
* The comma is used before a conjunction (and, but, for, or, nor, so, yet) linking two independent clauses:
Canadians watch America closely, but most Americans know little about Canada.
Exception: Some very brief independent clauses may not require a comma.
1. We dickered and then we made a deal. – Red Smith
2. I have seen the future and now I'm tired of it. – Gerald Nachman
* If one or both independent clauses have internal punctuation (especially commas), a writer might choose to separate two clauses with a semicolon and a coordinating conjunction so that the reader can easily see the main division of the sentence.
Genetically, we are nearly identical to mankind fifty thousand years ago; and some of us delight in the continuity represented by this, while others may be appalled. – Edward Hoagland
* Comma alone should be used between two independent clauses (comma splice):
"I plan to travel to England", my friend said happily. "I want to visit Shakespeare's birthplace."
* When a conjunction adverb joins the independent clauses in a compound sentence, it is preceded by a semicolon:
Petra was absent on Friday; consequently, she missed the chemistry test.
* The use of a comma to join coordinate clauses is more common in novels, stories, and some types of journalistic writing than it is in serious expository prose. Although it is hard to make general statements here, it is safe to say this practice is the exception, not the rule. The comma is used by most writers to join coordinate clauses in the following situation:
# When the series of sentences takes the form of a climax:
1. I came, I saw, I conquered.
2. The leaves are turning to gold, squirrels are fattening, hunting time is near.
# When the statements form an antithesis, or are arranged in the "it was not merely this, it was mainly that" formula:
1. It was more than an annoyance, it was a pang. – Winston S. Churchill
2. To allow the Mahdi to enter Khartoum would not merely mean to return the whole of the Sudan to barbarism, it would be a menace to the safety of Egypt herself. – Lyton Strachey