Colon (punctuation)

Punctuation mark consisting of two equally sized dots centered on the same vertical line

The colon : is a punctuation mark consisting of two equally sized dots centered on the same vertical line. A colon often precedes an explanation or a list. A colon is also used between hours and minutes, titles and subtitles of books, city and publisher in bibliographies, in biblical citations between chapter and verse, and for salutations in business letters and other formal letter writing.[1][failed verification]

The English word "colon" is from Latin colon (pl. cola), itself from Ancient Greek κῶλον (kôlon), meaning "limb", "member", or "portion". In Greek rhetoric and prosody, the term did not refer to punctuation but to the expression or passage itself. A "colon" was a section of a complete thought or passage. From this usage, in palaeography, a colon is a clause or group of clauses written as a line in a manuscript.[2] In the punctuation system devised by Aristophanes of Byzantium in the 3rd century BC, the end of such a clause was thought to occasion a medium-length breath and was marked by a middot·⟩. (This was only intermittently used, but eventually revived as the ano teleia, the modern Greek semicolon.[3]) A double dot symbol⟩, meanwhile, later came to be used as a full stop or to mark a change of speaker. A variant was introduced to English orthography around 1600, marking a pause intermediate between a comma and a full stop.[4] As late as the 18th century, the appropriateness of a colon was still being related to the length of the pause taken when reading the text aloud, but silent reading eventually replaced this with other considerations.[5]

In British English, it was once common for a colon to be followed by a hyphen or dash to indicate a restful pause, in a typographical construction known as the "dog's bollocks", though this usage is now discouraged.[6][7][8]

In modern English usage, a complete sentence precedes a colon, while a list, description, explanation, or definition follows it. The elements which follow the colon may or may not be a complete sentence: since the colon is preceded by a sentence, it is a complete sentence whether what follows the colon is another sentence or not. While it is acceptable to capitalize the first letter after the colon in American English, it is not the case in British English, except where a proper noun immediately follows a colon.[9]

Daequan was so hungry that he ate everything in the house: chips, cold pizza, pretzels and dip, hot dogs, peanut butter, and candy.
Bertha is so desperate that she'll date anyone, even William: he's uglier than a squashed toad on the highway, and that's on his good days. and criticism on it, I had to constantly look up the word "egregious" since the villain uses that word: outstandingly bad or shocking.
I guess I can say I had a rough weekend: I had chest pain and spent all Saturday and Sunday in the emergency room.

Some writers use fragments (incomplete sentences) before a colon for emphasis or stylistic preferences (to show a character's voice in literature), as in this example:

The Bedford Handbook describes several uses of a colon. For example, one can use a colon after an independent clause to direct attention to a list, an appositive or a quotation, and it can be used between independent clauses if the second summarizes or explains the first. In non-literary or non-expository uses, one may use a colon after the salutation in a formal letter, to indicate hours and minutes, to show proportions, between a title and subtitle, and between city and publisher in bibliographic entries.[10]

Luca Serianni, an Italian scholar who helped to define and develop the colon as a punctuation mark, identified four punctuational modes for it: syntactical-deductive, syntactical-descriptive, appositive, and segmental.[11]

The colon introduces the logical consequence, or effect, of a fact stated before.

In this sense the colon introduces a description; in particular, it makes explicit the elements of a set.

Syntactical-descriptive colons may separate the numbers indicating hours, minutes, and seconds in abbreviated measures of time.[12]

British English, however, more frequently uses a point for this purpose:

A colon is also used in the descriptive location of a book verse if the book is divided into verses, such as in the Bible or the Quran:

An appositive colon also separates the subtitle of a work from its principal title. Dillon has noted the impact of colons on scholarly articles,[15][16] but the reliability of colons as a predictor of quality or impact has also been challenged.[17][18] In titles, neither needs to be a complete sentence as titles do not represent expository writing:

Like a dash or quotation mark, a segmental colon introduces speech. The segmental function was once a common means of indicating an unmarked quotation on the same line. The following example is from the grammar book The King's English:

Benjamin Franklin proclaimed the virtue of frugality: A penny saved is a penny earned.

This form is still used in written dialogues, such as in a play. The colon indicates that the words following an individual's name are spoken by that individual.

Use of capitalization or lower-case after a colon varies. In British English, and in most Commonwealth countries, the word following the colon is in lower case unless it is normally capitalized for some other reason, as with proper nouns and acronyms. British English also capitalizes a new sentence introduced by colon's segmental use;

American English goes further and permits writers to similarly capitalize the first word of any independent clause following a colon. This follows the guidelines of some modern American style guides, including those published by the Associated Press and the Modern Language Association. The Chicago Manual of Style, however, requires capitalization only when the colon introduces a direct quotation, a direct question, or two or more complete sentences.[19]

In many European languages, the colon is usually followed by a lower-case letter unless the upper case is required for other reasons, as with British English. German usage requires capitalization of independent clauses following a colon.[20] Dutch further capitalizes the first word of any quotation following a colon, even if it is not a complete sentence on its own.[21]

In print, a thin space was traditionally placed before a colon and a thick space after it. In modern English-language printing, no space is placed before a colon and a single space is placed after it[citation needed]. In French-language typing and printing, the traditional rules are preserved.

One or two spaces may be and have been used after a colon. The older convention (designed to be used by monospaced fonts) was to use two spaces after a colon.[22]

In Finnish and Swedish, the colon can appear inside words in a manner similar to the apostrophe in the English possessive case, connecting a grammatical suffix to an abbreviation or initialism, a special symbol, or a digit (e.g., Finnish USA:n and Swedish USA:s for the genitive case of "USA", Finnish %:ssa for the inessive case of "%", or Finnish 20:een for the illative case of "20").

Written Swedish uses colons in contractions, such as S:t for Sankt (Swedish for "Saint") – for example in the name of the Stockholm metro station S:t Eriksplan. This can even occur in people's names, for example Antonia Ax:son Johnson (Ax:son for Axelson). Early Modern English texts also used colons to mark abbreviations.[23][24]

In Armenian, a colon indicates the end of a sentence, similar to a Latin full stop or period.

In Hebrew, the Sof pasuq is used in some writings such as prayer books to signal the end of a verse.

See also colon (letter) for the use of a colon-like character as an alphabetic character rather than as punctuation.

The colon is used in mathematics, cartography, model building, and other fields—in this context it denotes a ratio or a scale, as in 3:1 (pronounced "three to one"). When a ratio is reduced to a simpler form, such as 10:15 to 2:3, this may be expressed with a double colon as 10:15::2:3; this would be read "10 is to 15 as 2 is to 3". This form is also used in tests of logic where the question of "Dog is to Puppy as Cat is to _____?" can be expressed as "Dog:Puppy::Cat:_____".

In some languages (e.g. German, Russian and French), the colon is the commonly used sign for division (instead of ÷).

The notation ƒ: X → Y indicates that f is a function with domain X and codomain Y.

In mathematical logic, when using set-builder notation for describing the characterizing property of a set, it is used as an alternative to a vertical bar (which is the ISO 31-11 standard), to mean "such that". Example:

In older literature on mathematical logic, it is used to indicate how expressions should be bracketed (see Glossary of Principia Mathematica).

In type theory and programming language theory, the colon sign after a term is used to indicate its type, sometimes as a replacement to the "∈" symbol. Example:

A colon is also sometimes used to indicate a tensor contraction involving two indices, and a double colon (::) for a contraction over four indices.

A colon is also used to denote a parallel sum operation involving two operands (many authors, however, instead use a sign and a few even a for this purpose).

The character was on early typewriters and therefore appeared in most text encodings, such as Baudot code and EBCDIC. It was placed at code 58 in ASCII and from there inherited into Unicode. Unicode also defines several related characters.

A number of programming languages, most notably Pascal and Ada, use a colon immediately followed by an equals sign (:=) as the assignment operator, to distinguish it from a single equals which is an equality test (C instead used a single equals as assignment, and a double equals as the equality test).

Many languages including C, C++ and DOS batch files use the colon to indicate the text before it is a label, such as a target for a goto or an introduction to a case in a switch statement. In a related use, Python uses a colon to separate a control statement from the block of statements it controls:

In a number of languages, including JavaScript and Python, colons are used to define name-value pairs in a dictionary or object. This is also used by data formats such as JSON. Some other languages use an equals sign.

The colon is used as part of the ?: conditional operator in C and many other languages.

C++ uses a double colon as the scope resolution operator, and class member access. Most other languages use a period but C++ had to use this for compatibility with C.

In BASIC, it is used as a separator between the statements or instructions in a single line. Most other languages use a semicolon, but BASIC had used semicolon to separate items in print statements.

Haskell uses a colon (pronounced as "cons", short for "construct") as an operator to add an element to the front of a list:[28]

while a double colon :: is read as "has type of" (compare scope resolution operator):[29]

The ML languages (including Standard ML and OCaml) have the above reversed, where the double colon (::) is used to add an element to the front of a list; and the single colon (:) is used for type guards.

MATLAB uses the colon as a binary operator that generates vectors, as well as to select particular portions of existing matrices.

In the esoteric programming language INTERCAL, the colon is called "two-spot" and is used to identify a 32-bit variable—distinct from a spot (.) which identifies a 16-bit variable.

Internet URLs use the colon to separate the protocol (such as http:) from the IPv6 address.

In an IPv6 address colons (and one optional double colon) separate up to 8 groups of 16 bits in hexadecimal representation.[31] In a URL a colon follows the initial scheme name (such as HTTP and FTP), and separates a port number from the hostname or IP address.[32]

In Microsoft Windows filenames, the colon is reserved for use in alternate data streams and cannot appear in a filename. It was used as the directory separator in Classic Mac OS, and was difficult to use in early versions of the newer BSD-based macOS due to code swapping the slash and colon to try to preserve this usage. In most systems it is often difficult to put a colon in a filename as the shell interprets it for other purposes.

CP/M and early versions of MSDOS required the colon after the names of devices, such as CON: though this gradually disappeared except for disks (where it had to be between the disk name and the required path representation of the file as in C:\Windows\). This then migrated to use in URLs.[32]

It is often used as a single post-fix delimiter, signifying a token keyword had immediately preceded it or the transition from one mode of character string interpretation to another related mode. Some applications, such as the widely used MediaWiki, utilize the colon as both a pre-fix and post-fix delimiter.

In wiki markup, the colon is often used to indent text. Common usage includes separating or marking comments in a discussion as replies, or to distinguish certain parts of a text.

In human-readable text messages, a colon, or multiple colons, is sometimes used to denote an action (similar to how asterisks are used)[original research?] or to emote (for example, in vBulletin). In the action denotation usage it has the inverse function of quotation marks, denoting actions where unmarked text is assumed to be dialogue. For example:

Tom: Pluto is so small; it should not be considered a planet. It is tiny!
Mark: Oh really? ::drops Pluto on Tom's head:: Still think it's small now?

Colons may also be used for sounds, e.g., ::click::, though sounds can also be denoted by asterisks or other punctuation marks.