Colon (punctuation)


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.

History

The English word "colon" is from Latin colon, itself from Ancient Greek κῶλον, 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.
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. 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. 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.
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.

Usage in English

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.
;Colon used before list
;Colon used before a description
;Colon before definition
;Colon before explanation
Some writers use fragments before a colon for emphasis or stylistic preferences, 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.
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.

Syntactical-deductive

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

Syntactical-descriptive

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.
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:

Appositive

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

Segmental

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:
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 capitals

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.
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. Dutch further capitalizes the first word of any quotation following a colon, even if it is not a complete sentence on its own.

Spacing

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. 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 was to use two spaces after a colon.

Usage in other languages

Suffix separator

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.

Abbreviation mark

Written Swedish uses colons in contractions, such as S:t for Sankt – for example in the name of the Stockholm metro station S:t Eriksplan. This can even occur in people's names, for example . Early Modern English texts also used colons to mark abbreviations.

End of sentence or verse

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 for the use of a colon-like character as an alphabetic character rather than as punctuation.

Mathematics and logic

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. 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, the colon is the commonly used sign for division.
The notation | : | may also denote the index of a subgroup.
The notation indicates that is a function with domain and codomain.
The combination with an equal sign is used for definitions.
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, to mean "such that". Example:
In older literature on mathematical logic, it is used to indicate how expressions should be bracketed.
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.

Computing

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.
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:

if test:
print
else:
print

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.

var obj =

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.
Erlang uses a single colon for scope resolution.
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.
In Forth, a colon precedes definition of a new word.
Haskell uses a colon as an operator to add an element to the front of a list:
"child" : -- equals
while a double colon :: is read as "has type of" :
::
The ML languages 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.
APL uses the colon
The colon is also used in many operating systems commands.
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.

Addresses

Internet URLs use the colon to separate the protocol from the hostname or IP address.
In an IPv6 address colons separate up to 8 groups of 16 bits in hexadecimal representation. In a URL a colon follows the initial scheme name, and separates a port number from the hostname or IP address.
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 though this gradually disappeared except for disks. This then migrated to use in URLs.

Text markup

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 or to emote. 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:
Colons may also be used for sounds, e.g., ::click::, though sounds can also be denoted by asterisks or other punctuation marks.
Colons can also be used to represent eyes in emoticons.