Colon (punctuation)

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

There was only one possible explanation: the train had never arrived.

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

I have three sisters: Daphne, Rose, and Suzanne.

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

The concert begins at 21:45.

The rocket launched at 09:15:05.

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

The programme will begin at 8.00 pm.

You will need to arrive by 14.30.[13]

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:

"Isaiah 42:8"

"Deuteronomy 32:39"

"Quran 10:5"

Luruns could not speak: he was drunk.[14]

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:

Star Wars Episode VI: Return of the Jedi

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.

Patient: Doctor, I feel like a pair of curtains.

Doctor: Pull yourself together!

Use of capitalization or lower-case after a colon varies. In British English, 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. 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.

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:

if test(x):
  print "test(x) is true!"
else:
  print "test(x) is not true..."

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 = {
    name: "Charles",
    age: 18,
}

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.

In Forth, a colon precedes definition of a new word.

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

"child" : ["woman", "man"] -- equals ["child","woman","man"]

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

("text", False) :: ([Char], Bool)

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.

APL uses the colon[30]

• to introduce a control structure element. In this usage it must be the first non-blank character of the line.
• after a label name that will be the target of a :goto or a right-pointing arrow (Note: this style of programming is deprecated and programmers are encouraged to use control structures instead).
• to separate a guard (boolean expression) from its expression in a dynamic function. Two colons are used for an Error guard (one or more error numbers).
• Colon + space are used in class definitions to indicate inheritance.
• ⍠ (a colon in a box) is used by APL for its variant operator.[30]

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.

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.

Normal text.
:Dented text by the means of a colon.
::The gap increases with colon number.

In human-readable text messages, a colon, or multiple colons, is sometimes used to denote an action (similar to how asterisks are used) 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.

Colons can also be used to represent eyes in emoticons.