The comma is a punctuation mark that appears in several variants in different 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. Some typefaces render it as a small line, slightly curved or straight, but inclined from the vertical. Other fonts give it the appearance of a miniature filled-in figure on the baseline.
The comma is used in many contexts and languages, mainly to separate parts of a sentence such as clauses, and items in lists mainly when there are three or more items listed. The word comma comes from the Greek (), which originally meant a cut-off piece, specifically in grammar, a short clause.
A comma-shaped mark is used as a diacritic in several writing systems and is considered distinct from the cedilla. In Byzantine and modern copies of Ancient Greek, the "rough" and "smooth breathings" () appear above the letter. In Latvian, Romanian, and Livonian, the comma diacritic appears below the letter, as in .
In spoken language, a common rule of thumb is that the function of a comma is generally performed by a pause.
For the notation and /x/ used in this article, see grapheme and phoneme, respectively.
History
The development of punctuation is much more recent than the alphabet.
In the 3rd century BC, Aristophanes of Byzantium invented a system of single dots () at varying levels, which separated verses and indicated the amount of breath needed to complete each fragment of the text when reading aloud. The different lengths were signified by a dot at the bottom, middle, or top of the line. For a short passage, a in the form of a dot was placed mid-level. This is the origin of the concept of a comma, although the name came to be used for the mark itself instead of the clause it separated.
The mark used today is descended from a , a diagonal slash known as , used from the 13th to 17th centuries to represent a pause. The modern comma was first used by Aldus Manutius.
Uses in English
In general, the comma shows that the words immediately before the comma are less closely or exclusively linked grammatically to those immediately after the comma than they might be otherwise. The comma performs a number of functions in English writing. It is used in generally similar ways in other languages, particularly European ones, although the rules on comma usage – and their rigidity – vary from language to language.
Serial (or Oxford) comma
Commas are placed between items in lists, as in They own a cat, a dog, two rabbits, and seven mice.
Whether the final conjunction, most frequently and, should be preceded by a comma, called the serial comma, is one of the most disputed linguistic or stylistic questions in English:
They served apples, peaches, and bananas. (serial comma used) We cleaned up cores, pits and skins. (serial comma omitted)
The serial comma is used much more often, usually routinely, in the United States. A majority of American style guides mandate its use, including The Chicago Manual of Style, Strunk and White's classic The Elements of Style, and the U.S. Government Printing Office Style Manual. Conversely, the AP Stylebook for journalistic writing advises against it.
The serial comma is also known as the Oxford comma, Harvard comma, or series comma. Although less common in British English, its usage occurs within both American and British English. It is called the Oxford comma because of its long history of use by Oxford University Press.
According to New Hart's Rules, "house style will dictate" whether to use the serial comma. "The general rule is that one style or the other should be used consistently." No association with region or dialect is suggested, other than that its use has been strongly advocated by Oxford University Press. Its use is preferred by Fowler's Modern English Usage. It is recommended by the United States Government Printing Office, Harvard University Press, and the classic Elements of Style of Strunk and White.
Use of a comma may prevent ambiguity:
The sentence I spoke to the boys, Sam and Tom could mean either I spoke to the boys and Sam and Tom (I spoke to more than three people) or I spoke to the boys, who are Sam and Tom (I spoke to two people); I spoke to the boys, Sam, and Tom – must be the boys and Sam and Tom (I spoke to more than three people).