Comma (,)

The comma ( , ) is a punctuation mark that appears in several variants in various languages. It has the same shape as an apostrophe or single closing quotation mark in many typefaces, but it differs from them in being placed on the baseline of the text.

Commas - The most common punctuation mark

The comma (,) is one of the most frequently used punctuation marks in English and various other languages. Depending on the typeface, a comma might resemble an apostrophe or single quotation mark, except a comma appears at the baseline of letters. The shape of a comma can range from a tiny, curved line to a miniature, blacked-out numeral 9.

In most languages, the comma is primarily used to split sentences into clauses and to divide items within lists of three or more things. The word "comma" derives from the Greek word komma, which refers to a brief clause or a sentence cut short. 

Using the comma correctly

In English and most other European languages, the comma fulfills a variety of functions, but is generally used to inform readers that the words on each side of the mark serve distinct purposes within a given sentence. The most common example is the separation of clauses:

I've never been to Australia, but someday I'd love to go to the Sydney Opera House.

The Empire State Building is a marvel of architecture, but one day it will have to be demolished.

Commas are also used after sentence-starting adverbs like therefore, furthermore, and however:

Nevertheless, he's been a formidable presence on the campaign trail.

Still, I don't think you should try that stunt.

When it comes to lists of more than two people or things within a given sentence, commas are placed after each one: 

I saw Nicole, Paul, Damian, Aristae, and Spring at the club last night.

Never leave home without your keys, debit cards, and Kleenex.

Background information on the term

Controversy lingers around the use of a comma before the coordinate "and" that precedes the final item in a list within a sentence; this is known as a serial comma. Traditionally, Americans have been taught not to place commas in front of "and," yet the serial comma is the standard for most academic research and writing and advocated by both the Oxford University Press and the Harvard University Press.

The following two versions of the same sentence illustrate the benefits of using serial commas:

I advised the girls, Betty and Judy to keep pepper spray on their key chains.

I advised the girls, Betty, and Judy to keep pepper spray on their key chains.

In the first version, it's hard to tell whether there's three entities being discussed—a group of girls, plus two other females—or if Betty and Judy are "the girls." In order for the latter possibility to work, however, a comma is needed after Judy; that would make the two names a subordinate clause of the word "girls." As a matter of fact, the sentence concerns a group of young girls and two adult women, and the second version conveys that much more clearly.

The use of a serial comma won't prevent confusion in all cases. If the sentence had said "I advised the girl, Betty, and Judy to keep pepper spray on their key chains," it would be unclear whether three people are being mentioned, or if Betty is "the girl." The best solution in this case would be to rewrite the first part of that sentence in the following order: "I advised Betty, Judy, and the girl."

In sentences that list multiple entities with commas in their names, semicolons should be used to separate each entity:

Blood, Sweat & Tears; Pete Brown & Piblokto!; Supply, Demand & Curve; and Hatfield & the North each contributed to the melding of rock and jazz.