>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, or with the appearance of a small, filled-in figure 9.

>The comma is used in many contexts and languages, mainly for separating parts of a sentence such as clauses, and items in lists, particularly when there are three or more items listed. The word comma comes from the Greek κόμμα (kómma), which originally meant a cut-off piece; specifically, in grammar, a short clause.[1][2]

A comma-shaped mark is used as a diacritic in several writing systems, and is considered distinct from the cedilla. The rough and smooth breathings (ἁ, ἀ) appear above the letter in Ancient Greek, and the comma diacritic appears below the letter in Latvian, Romanian, and Livonian.

In the 3rd century BC, Aristophanes of Byzantium invented a system of single dots (distinctiones) that separated verses (colometry) and indicated the amount of breath needed to complete each fragment of the text when reading aloud.[3] The different lengths were signified by a dot at the bottom, middle, or top of the line. For a short passage (a komma), a media distinctio 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 diagonal slash, or virgula suspensiva ( / ), used from the 13th to 17th centuries to represent a pause. The modern comma was first used by Aldus Manutius.[4][5]

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.

In lists
Commas are placed between items in lists, as in They own a cat, a dog, two rabbits, and seven mice. Some English style guides recommend that a comma be used before the final conjunction (and, or, nor) in a list of more than two elements. A comma used in such a position is variously called a serial comma, an Oxford comma, or a Harvard comma (after the Oxford University Press and Harvard University Press, both prominent advocates of this style). Such use of a comma sometimes prevents 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.
As a rule of thumb, The Guardian Style Guide[6] 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:[7]

>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 those trees over 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.[8][9] 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 not an independent 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,[8][9] as in:

>Sit down and shut up.
The above guidance is not universally accepted or applied. Long coordinating clauses are nonetheless usually separated by commas:[10]

>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 usage 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;[11] in most cases a semicolon should be used instead. A comma splice should not be confused, though, with asyndeton, a literary device used for a specific effect in which coordinating conjunctions are purposely omitted.

>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 below examples, if the two sentences are separated by a semicolon and the second sentence starts with an adverb, then it is preceded by a semicolon and followed by a comma.

In this sentence, furthermore, commas would also be called for.
This sentence is similar; however, a semicolon 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.[12]
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.
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[13] and the AP Stylebook,[14] 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:[15] “;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[16] and the AP Stylebook,[17] recommend that the second element be treated as a parenthetical, requiring a second comma after: “;The plane landed in Kampala, Uganda, that evening.”[18]

>The United States Postal Service[19] and Royal Mail[20] recommend leaving out punctuation when writing addresses on actual letters and packages, as the marks hinder optical character recognition.

In numbers
>Main article: Decimal mark
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 five or four 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. In some styles, the comma may not be used for this purpose at all (e.g. in the SI writing style[21]); a space may be used to separate groups of three digits instead.

In names
Commas are used when writing names that are presented 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.