The comma is a punctuation mark that appears in several variants in different languages. Some typefaces render it as a small line, slightly curved or straight, but inclined from the vertical; others give it the appearance of a miniature filled-in figure placed on the baseline. In many typefaces it is the same shape as an apostrophe or single closing quotation mark .
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 this article, denotes a grapheme (writing) and /x/ denotes a phoneme (sound).
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., Haverford CollegeManuscript Studies, Medieval and Early Modern – Palaeography: Punctuation glossary
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.
List separator and the serial (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) They served apples, peaches and bananas. (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 Publishing Office's 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 and Harvard University Press.
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).
The serial comma does not eliminate all confusion. Consider the following sentence: I thank my mother, Anne Smith, and Thomas. This could mean either my mother and Anne Smith and Thomas (three people) or my mother, who is Anne Smith; and Thomas (two people). This sentence might be recast as "my mother (Anne Smith) and Thomas" for clarity. I thank my mother, Anne Smith and Thomas. Because the comma after "mother" is conventionally used to prepare the reader for an appositive phrase – that is, a renaming of or further information about a noun – this construction formally suggests that my mother's name is "Anne Smith and Thomas". Because that is implausible, it is relatively clear that the construction refers to two separate people. Compare "I thank my friend, Smith and Wesson", in which the ambiguity is obvious to those who recognise Smith and Wesson as a business name.
As a rule of thumb, The Guardian Style Guide suggests that straightforward lists (he ate ham, eggs and chips) do not need a comma before the final "and", but sometimes it can help the reader (he ate cereal, kippers, bacon, eggs, toast and marmalade, and tea). The Chicago Manual of Style and other academic writing guides require the serial comma: all lists must have a comma before the "and" prefacing the last item in a series .
If the individual items of a list are long, complex, affixed with description, or themselves contain commas, semicolons may be preferred as separators, and the list may be introduced with a colon.
In news headlines, a comma might replace the word "and", even if there are only two items, in order to save space, as in this headline from Reuters: Trump, Macron engage in a little handshake diplomacy.
Separation of clauses Commas are often used to separate clauses. In English, a comma is often used to separate a dependent clause from the independent clause if the dependent clause comes first: After I fed the cat, I brushed my clothes. (Compare this with I brushed my clothes after I fed the cat.) A relative clause takes commas if it is non-restrictive, as in I cut down all the trees, which were over six feet tall. (Without the comma, this would mean that only the trees more than six feet tall were cut down.) Some style guides prescribe that two independent clauses joined by a coordinating conjunction (for, and, nor, but, or, yet, so) must be separated by a comma placed before the conjunction. In the following sentences, where the second clause is independent (because it can stand alone as a sentence), the comma is considered by those guides to be necessary: Mary walked to the party, but she was unable to walk home. Designer clothes are silly, and I can't afford them anyway. Don't push that button, or twelve tons of high explosives will go off right under our feet! In the following sentences, where the second half of the sentence is a dependent clause (because it does not contain an explicit subject), those guides prescribe that the comma be omitted: Mary walked to the party but was unable to walk home. I think designer clothes are silly and can't afford them anyway. However, such guides permit the comma to be omitted if the second independent clause is very short, typically when the second independent clause is an imperative, as in: Sit down and shut up. The above guidance is not universally accepted or applied. Long coordinate clauses, particularly when separated by "but", are often separated by commas: She had very little to live on, but would never have dreamed of taking what was not hers.
The joining of two independent sentences with a comma and no conjunction (as in "It is nearly half past five, we cannot reach town before dark.") is known as a comma splice and is sometimes considered an error in English; in most cases a semicolon should be used instead. A comma splice should not be confused, though, with the literary device called asyndeton, in which coordinating conjunctions are purposely omitted for a specific stylistic effect.
A much debated comma is the one in the Second Amendment to the United States Constitution, which says "A well regulated Militia being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." but ratified by several states as "A well regulated Militia being necessary to the security of a free State, the right of the people to keep and bear Arms shall not be infringed." which has caused much debate on its interpretation.
Certain adverbs Commas are always used to set off certain adverbs at the beginning of a sentence, including however, in fact, therefore, nevertheless, moreover, furthermore, and still.
Therefore, a comma would be appropriate in this sentence. Nevertheless, I will not use one.
If these adverbs appear in the middle of a sentence, they are followed and preceded by a comma. As in the second of the two examples below, if a semicolon separates the two sentences and the second sentence starts with an adverb, this adverb is preceded by a semicolon and followed by a comma.
In this sentence, furthermore, commas would also be called for. This sentence is a bit different; however, a comma is necessary as well.
Using commas to offset certain adverbs is optional, including then, so, yet, instead, and too (meaning also).
So, that's it for this rule. or So that's it for this rule. A comma would be appropriate in this sentence, too. or A comma would be appropriate in this sentence too.
Parenthetical phrases
Commas are often used to enclose parenthetical words and phrases within a sentence (i.e., information that is not essential to the meaning of the sentence). Such phrases are both preceded and followed by a comma, unless that would result in a doubling of punctuation marks or the parenthetical is at the start or end of the sentence. The following are examples of types of parenthetical phrases: Introductory phrase: Once upon a time, my father ate a muffin.Garner's Modern American Usage, (Oxford: 2003, p. 655) Interjection: My father ate the muffin, gosh darn it! Aside: My father, if you don't mind me telling you this, ate the muffin. Appositive: My father, a jaded and bitter man, ate the muffin. Absolute phrase: My father, his eyes flashing with rage, ate the muffin. Free modifier: My father, chewing with unbridled fury, ate the muffin. Resumptive modifier: My father ate the muffin, a muffin which no man had yet chewed. Summative modifier: My father ate the muffin, a feat which no man had attempted.
The parenthesization of phrases may change the connotation, reducing or eliminating ambiguity. In the following example, the thing in the first sentence that is relaxing is the cool day, whereas in the second sentence, it is the walk since the introduction of commas makes "on a cool day" parenthetical:
They took a walk on a cool day that was relaxing.
They took a walk, on a cool day, that was relaxing.
As more phrases are introduced, ambiguity accumulates, but when commas separate each phrase, the phrases clearly become modifiers of just one thing. In the second sentence below, that thing is the walk:
They took a walk in the park on a cool day that was relaxing.
They took a walk, in the park, on a cool day, that was relaxing.
Between adjectives A comma is used to separate coordinate adjectives (i.e., adjectives that directly and equally modify the following noun). Adjectives are considered coordinate if the meaning would be the same if their order were reversed or if and were placed between them. For example: The dull, incessant droning but the cute little cottage.