comma

1.  Commas are used to separate items in a list. When there are only two items, use no comma:

Hilda was back in a few moments wearing a long gray squirrel coat and a broad fur hat.

When there are more than two items, use commas between each item:

. . . two cotton handkerchiefs, some brass trinkets, and a little tobacco.

The last comma in a list, known as the serial comma, is controversial. Omitting the serial comma can lead to ambiguity:

He introduced his friends, his wife and his daughter.

But some style guides say the serial comma is superfluous where there’s no risk of ambiguity. This is standard practice for many news organizations, including the New York Times:

. . . artworks by Shirin Neshat, Alex Katz and Yoko Ono.

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.

The lack of comma in “clean and safe” leaves the sentence open to misreadings. What’s clean and safe — the bed or the guests?

2.  Commas link independent clauses in a compound sentence:

Andy Warhol was a big fan, and so was Roy Lichtenstein.

The metal trappings of the harnesses shone dazzlingly, and the wheels were revolving disks that threw off rays of light.

3.  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.

4.  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.

5.  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.

6.  A comma separates a quotation from its attribution:

“Sunday afternoon, then,” said Alexander, as she rose to join her hostess.

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.”

7.  According to conventional comma usage rules, a comma comes after a salutation and complimentary close in informal letters:

Dear Debbie,

Yours sincerely,

In formal letters, a colon is used.

8.  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.

9.  A comma separates elements in an address:

188 7th Avenue, Brooklyn, NY