In modern English, the comma ( , ) has a few conventional uses.
Uses of commas
Listing
Commas are used to separate items in a list. When there are only two items, there is no comma:
Hilda was back in a few moments wearing a long gray squirrel coat and a broad fur hat. [Alexander's Bridge, Willa Cather]
When there are more than two items, use commas between each item:
… two cotton handkerchiefs, some brass trinkets, and a little tobacco. [The Voyage of the Beagle, Charles Darwin]
The last comma in a list, known as the serial comma or the Oxford comma, is controversial. Omitting the serial comma can lead to ambiguity:
He introduced his friends, Diana and Susan.
When we read this sentence out of context, there is no way to know whether it names Diana and Susan as the friends or lists Diana and Susan in addition to his friends.
But some style guides say the serial comma is superfluous where there’s no risk of ambiguity. Moreover, in the UK and Australia it’s common to find the serial comma regarded as an Americanism. It isn’t. In fact, it is also called the Oxford comma and non-American writers shouldn’t be afraid to use it for clarity.
Omitting the serial or Oxford comma is standard practice for many news organizations, including the New York Times, for example:
… artworks by Shirin Neshat, Alex Katz and Yoko Ono. [New York Times]
But some cases are not so clear cut:
On Christmas, the shelter lets guests sleep as late as they like, in beds that are warm, clean and safe. [New York Times]
The lack of comma in “clean and safe” leaves the sentence open to misreadings. What’s clean and safe—the bed or the guests?
Linking clauses
Commas link independent clauses in a compound sentence:
Andy Warhol was a big fan, and so was Roy Lichtenstein. [New York Times]
The metal trappings of the harnesses shone dazzlingly, and the wheels were revolving disks that threw off rays of light. [Alexander's Bridge, Willa Cather]
Parenthetical content
Commas set apart parenthetical content within a sentence:
Tracy Justice, a spokeswoman for the University of Michigan Health System Ann Arbor, confirmed that the hospital had received one patient from the flight. [LA Times]
Introductory phrases
A comma separates an introductory phrase from the main body of a sentence:
Between them, the two countries control 90 to 95 percent of the world’s nuclear weapons. [Boston Globe]
Multiple adjectives
Commas separate multiple adjectives modifying a noun:
We went down to the old, smelly station.
Where substituting an and for the comma would sound wrong, no comma is necessary:
… the ailing former president.
Setting off quotes
A comma separates a quotation from its attribution:
“Sunday afternoon, then,” said Alexander, as she rose to join her hostess. [Alexander's Bridge, Willa Cather]
However, there is no comma where the quotation fits into the syntax of the sentence:
President Barack Obama said Thursday that the country is close to the “end of a nearly century-long struggle to reform America’s health care system.” [CNN]
Greetings and closings
According to the conventional rules, a comma comes after a salutation and complimentary close in informal letters:
Dear Debbie,
Yours sincerely,
In formal letters, a colon is used.
Dates
A comma goes between the day and year in the date construction that looks like, “July 2, 1980.” No comma is used in the construction, “2 July 1980.” This comma-free form is popular in online publication, where the anti-comma bias is strong.
Addresses
Commas separate elements in addresses:
188 7th Avenue, Brooklyn, NY
