The colon ( : ) is a punctuation mark consisting of two equally sized dots centered on the same vertical line. A colon precedes an explanation or an enumeration, or list. A colon is also used with ratios, titles and subtitles of books, city and publisher in bibliographies, biblical citations between chapter and verse, and for salutations in business letters and other formal letter writing, and often to separate hours and minutes.
The use of a colon-like character as an alphabetic letter rather than as punctuation is covered at colon (letter).
The most common use of the colon is to inform the reader that what follows the colon proves, explains, defines, describes, or lists elements of what preceded it. 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.Williams was so hungry he ate everything in the house: chips, cold pizza, pretzels and dip, hot dogs, peanut butter, and candy.Jane is so desperate that she'll date anyone, even Tom: 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 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.
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. Although Serianni wrote this guide for the Italian language, his definitions apply also to English and many other languages.
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.
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 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.
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.
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.
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.
Beside usage similar to that of English, the colon has other functions. Several compatibility forms for Chinese and Japanese typography are encoded in Unicode:
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.
See also colon (letter) for the use of a colon-like character as an alphabetic character rather than as punctuation.
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. 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.) 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.
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:_____". Unicode provides a distinct character U+2236 ∶ RATIO for mathematical usage. 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).
Some languages like Haskell use a double colon (::) to indicate type instead.
A colon is also sometimes used to indicate a tensor contraction involving two indices, and a double colon (::) for a contraction over four indices.
Additional colon-related symbols for math are encoding in Unicode:
In Microsoft Windows filenames, the colon is reserved for use in alternate data streams. By default, however, filenames use the Segoe UI font, and the tone letter U+A789 is identical to the colon in this font, so it can be used in place of the colon.
A number of programming languages, most notably Pascal and Ada, use a colon immediately followed by an equality sign (
:=), in which case the colon and the equality sign are considering to compose to an independent assignment sign; this can be represented in Unicode as
U+2254 ≔ COLON EQUALS.
The colon is also used as part of the ?: conditional operator in C and other languages.
The colon is also used in many operating systems commands. 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.
The colon is quite often used as a special control character in URLs, computer programming languages, in the path representation of several file systems (such as FAT, following the drive letter, as in
C:\Windows\, and HFS).
In an IPv6 address colons (and one optional double colon) separate up to 8 groups of 16 bits in hexadecimal representation. 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.
In BASIC, it is used as a separator between the statements or instructions in a single line, which is represented in other languages via the semicolon.
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 Python, which uses indentation to indicate blocks, the colon is used in statements to indicate that the next line is the start of an indented block.
U+2360 ⍠ APL FUNCTIONAL SYMBOL QUAD COLON is used by APL for its variant operator.
On the Internet, 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:
Colons may also be used for sounds, e.g., ::click::, though sounds can also be denoted by asterisks or other punctuation marks.