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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 .

''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.

The serial 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).

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 apposite phrase – that is, a renaming of or further information about a noun – this construction suggests that my mother's name is "Anne Smith and Thomas". 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 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 are nonetheless usually separated by commas:
* ''She had very little to live on, but she would never have dreamed of taking what was not hers.''

In some languages, such as German and Polish, stricter rules apply on comma use between clauses, with dependent clauses always being set off with commas, and commas being generally proscribed before certain coordinating conjunctions.

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.''
*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.''
*''The devious lazy red frog'' suggests there are lazy red frogs (one of which is devious), while ''the devious, lazy red frog'' does not carry this connotation.

Before quotations

Some writers precede quoted material that is the grammatical object of an active verb of speaking or writing with a comma, as in ''Mr. Kershner says, "You should know how to use a comma."'' Quotations that follow and support an assertion are often preceded by a colon rather than a comma.

Other writers do not put a comma before quotations unless one would occur anyway. Thus they would write ''Mr. Kershner says "You should know how to use a comma."''

In dates

Month day, year

When a date is written as a month followed by a day followed by a year, a comma separates the day from the year: December 19, 1941. This style is common in American English. The comma is used to avoid confusing consecutive numbers: December 19 1941.
Most style manuals, including '' The Chicago Manual of Style''
and the ''AP Stylebook'',
also recommend that the year be treated as a parenthetical, requiring a second comma after it: ''"Feb. 14, 1987, was the target date."''

If just month and year are given, no commas are used: "Her daughter April may return in June 2009 for the reunion."

Day month year

When the day precedes the month, the month name separates the numeric day and year, so commas are not necessary to separate them: "The Raid on Alexandria was carried out on 19 December 1941."

In geographical names

Commas are used to separate parts of geographical references, such as city and state (''Dallas, Texas'') or city and country (''Kampala, Uganda''). Additionally, most style manuals, including '' The Chicago Manual of Style''
and the ''AP Stylebook'',
recommend that the second element be treated as a parenthetical, requiring a second comma after: ''"The plane landed in Kampala, Uganda, that evening."''

The United States Postal Service and Royal Mail recommend leaving out punctuation when writing addresses on actual letters and packages, as the marks hinder optical character recognition. Canada Post has similar guidelines, only making very limited use of hyphens.

In mathematics

Similar to the case in natural languages, commas are often used to delineate the boundary between multiple mathematical objects in a list (e.g., $(3, 5, 12)$). Commas are also used to indicate the comma derivative of a tensor.

In numbers

In representing large numbers, from the right side to the left, English texts usually use commas to separate each group of three digits in front of the decimal. This is almost always done for numbers of six or more digits, and often for four or five digits but not in front of the number itself. However, in much of Europe, Southern Africa and Latin America, periods or spaces are used instead; the comma is used as a decimal separator, equivalent to the use in English of the decimal point. In India, the groups are two digits, except for the rightmost group, which is of three digits. In some styles, the comma may not be used for this purpose at all (e.g. in the SI writing style); a space may be used to separate groups of three digits instead.

In names

Commas are used when rewriting names to present the surname first, generally in instances of alphabetization by surname: ''Smith, John.'' They are also used before many titles that follow a name: ''John Smith, Ph.D.''

Similarly in lists that are presented with an inversion: ''...; socks, green: 3 pairs; socks, red: 2 pairs; tie, regimental: 1''.

Ellipsis

Commas may be used to indicate that a word, or a group of words, has been omitted, as in ''The cat was white; the dog, brown.'' (Here the comma replaces ''was''.)

Vocative

Commas are placed before, after, or around a noun or pronoun used independently in speaking to some person, place, or thing:
*''I hope, John, that you will read this.''

Between the subject and predicate

In his 1785 essay ''An Essay on Punctuation'', Joseph Robertson advocated a comma between the subject and predicate of long sentences for clarity; however, this usage is regarded as an error in modern times.
*''The good taste of the present age, has not allowed us to neglect the cultivation of the English language.''
*''Whoever is capable of forgetting a benefit, is an enemy to society.''

Differences between American and British usage in placement of commas and quotation marks

The comma and the quotation mark can be paired in several ways.

In Great Britain and many other parts of the world, punctuation is usually placed within quotation marks only if it is part of what is being quoted or referred to:
* My mother gave me the nickname "Bobby Bobby Bob Bob Boy", which really made me angry.

In American English, the comma was commonly included inside a quotation mark:
* My mother gave me the nickname "Bobby Bobby Bob Bob Boy," which really made me angry.
However, this practice has fallen out of use in favor of the British form.

During the Second World War, the British carried the comma over into abbreviations. Specifically, "Special Operations, Executive" was written "S.O.,E.". Nowadays, even the full stops are frequently discarded.

Languages other than English

Western European languages

Western European languages like German, French, Italian, Spanish, and Portuguese use the same comma as English, with similar spacing, though usage may be somewhat different. For instance, in Standard German, subordinate clauses are always preceded by commas.

Comma variants

The basic comma is defined in Unicode as , and many variants by typography or language are also defined.

:

Some languages use a completely different sort of character for the purpose of the comma.

:

There are also a number of comma-like diacritics with "" in their Unicode names that are not intended for use as punctuation. A comma-like low quotation mark is also available (shown below; corresponding sets of raised single quotation marks and double-quotation marks are not shown).

:

There are various other Unicode characters that include commas or comma-like figures with other characters or marks, that are not shown in these tables.

Languages other than Western European

Korean punctuation uses both commas and interpuncts for lists.

In Unicode 5.2.0 "numbers with commas" ( through ) were added to the Enclosed Alphanumeric Supplement block for compatibility with the ARIB STD B24 character set.

Hebrew script is also written from right to left. However, Hebrew punctuation includes only a regular comma ().

Dravidian languages such as Tamil, Telugu, Kannada, and Malayalam also use the punctuation mark in similar usage to that of European languages with similar spacing. :ta:கால்புள்ளி (தமிழ் நடை)

Computing

In the common character encoding systems Unicode and ASCII, character 44 ( 0x2C) corresponds to the comma symbol. The HTML numeric character reference is ,.

In many computer languages commas are used as a field delimiter to separate arguments to a function, to separate elements in a list, and to perform data designation on multiple variables at once.

In the C programming language the comma symbol is an operator which evaluates its first argument (which may have side-effects) and then returns the value of its evaluated second argument. This is useful in ''for'' statements and macros.

In Smalltalk and APL, the comma operator is used to concatenate collections, including strings. In APL, it is also used monadically to rearrange the items of an array into a list.

In Prolog, the comma is used to denote Logical Conjunction ("and").

The comma-separated values (CSV) format is very commonly used in exchanging text data between database and spreadsheet formats.

Diacritical usage

The comma is used as a diacritic mark in Romanian under (, ), and under (, ). A cedilla is occasionally used instead of it, but this is technically incorrect. The symbol (' d with comma below') was used as part of the Romanian transitional alphabet (19th century) to indicate the sounds denoted by the Latin letter or letters , where derived from a Cyrillic ѕ (, ). The comma and the cedilla are both derivative of (a small cursive ) placed below the letter. From this standpoint alone, , , and could potentially be regarded as stand-ins for /sz/, /tz/, and /dz/ respectively.

In Latvian, the comma is used on the letters , , , , and historically also , to indicate palatalization. Because the lowercase letter has a descender, the comma is rotated 180° and placed over the letter. Although their Adobe glyph names are 'letter with comma', their names in the Unicode Standard are 'letter with a cedilla'. They were introduced to the Unicode standard before 1992 and, per Unicode Consortium policy, their names cannot be altered.

In Livonian, whose alphabet is based on a mixture of Latvian and Estonian alphabets, the comma is used on the letters , , , , to indicate palatalization in the same fashion as Latvian, except that Livonian uses and to represent the same palatal plosive phonemes which Latvian writes as and respectively.

In Czech and Slovak, the diacritic in the characters , , and resembles a superscript comma, but it is used instead of a caron because the letter has an ascender. Other ascender letters with carons, such as letters (used in Finnish Romani and Lakota) and (used in Skolt Sami), did not modify their carons to superscript commas.

In 16th-century Guatemala, the archaic letter cuatrillo with a comma ( and ) was used to write Mayan languages.

See also

* Hebrew cantillation
*Copy editing
* English punctuation
*Latin-script alphabet
* List of typographical symbols and punctuation marks
*Ogonek
* Part of speech
* Sentence clause structure
* Traditional grammar

Related history

* Global spread of the printing press
* History of printing in East Asia
*History of sentence spacing
* History of Western typography

References

External links

''Grammar, Punctuation, and Capitalization'' – a comprehensive online guide by NASA
– a satirical suggestion to settle the problem of the Oxford Comma once and for all

– another satirical compromise between the American and British traditions relating to quotes and commas.

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Punctuation
Greek-script diacritics
Latin-script diacritics