Colon (punctuation)

The colon is a punctuation mark consisting of two equally sized dots centered on the same vertical line. A colon is used to explain or start an enumeration. A colon is also used with ratios, titles and subtitles of books, city and publisher in biographies, hours and minutes, and formal letters.

In Unicode, it is encoded at U+003A : COLON (:).

Usage

The most common use of the colon is to inform the reader that what follows the colon proves, explains, or lists elements of what preceded it.

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. Also, it can be used between independent clauses if the second summarizes or explains the first. Furthermore, 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.

Syntactical-deductive

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

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

Syntactical-descriptive

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.

Similarly, the syntactical-descriptive colon separates chapter and verse numbers in citations of passages in widely-studied texts, such as epic poetry, religious texts, and the plays of William Shakespeare.

John 3:14–16 or John III:14–16 refers to verses 14 to 16 of chapter three of the Gospel of John.

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

The concert begins at 21:45.

The rocket launched at 09:15:05.

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

The programme will begin at 8.00 pm.

You will need to arrive by 14.30.

Appositive

The colon introduces an appositive independent clause. In other words, the sentence after the colon is in apposition to the one before the colon.

Bob could not speak: He was drunk.

An appositive colon also separates the subtitle of a work from its principal title.

Star Wars Episode IV: A New Hope

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:

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 capitals

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 a proper noun or an acronym, or is normally capitalized for some other reason (e.g. see segmental use hereinbefore). However, in American English, many writers capitalize the word following a colon if it begins an independent clause, i.e. a clause that could stand as a complete sentence. 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 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, such as for a proper noun). However, usage differs from this in German, where an upper-case letter may be used only if the sentence after the colon could stand alone without the preceding sentence (otherwise one may judge freely according to the relative independence of the two assertions), and in Dutch, where an upper-case letter must be used if the colon is followed by a quotation or an enumeration of complete sentences, although in all other cases a lower-case letter should be used.

Spacing

In print, a thin space is traditionally placed before a colon and a thick space after it. In English-language modern high-volume commercial 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. See also Double spacing at the end of sentences.

History

Further information: Colon (rhetoric)

English colon is from Latin colon (plural cola), itself from Greek κῶλον "limb, member, portion", in rhetoric or prosody especially a part or section of a sentence or a rhythmical period of an utterance. In palaeography, a colon is a clause or group of clauses written as a line.

The OED cites William Blades' The life and typography of W. Caxton (1882), p. 126:

"The Greek grammarians […] called a complete sentence a period, a limb was a colon, and a clause a comma."

Use of the : symbol to mark the discontinuity of a grammatical construction, or a pause of a length intermediate between that of a semicolon and that of a period, was introduced in English orthography around 1600.

John Bullokar's An English expositor (1616) glosses Colon as "A marke of a sentence not fully ended which is made with two prickes."

John Mason in An essay on elocution (1748) prescribes "A Comma Stops the Voice while we may privately tell one, a Semi Colon two; a Colon three: and a Period four."

IPA Diacritical usage

The IPA length mark

A special triangular colon symbol is used in IPA to indicate that the preceding sound is long. Its form is that of two triangles, each a little larger than a point (dot) of a standard colon, pointing toward each other. It is available in Unicode as modifier letter triangular colon, Unicode U+02D0 (ː). A regular colon is often used as a fallback when this character is not available, and in the practical orthography of some languages which have a phonemic long/short distinction in vowels.

If the upper triangle is used without the lower one, it designates a "half-long" vowel.

Word-medial 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 (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").

Abbreviation

In Swedish, the colon is used in contractions, such as S:t for Sankt (Swedish for "Saint"), e.g. in 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). The colon was also used to mark abbreviations in early modern English.

Letter

The colon is also used as a grammatical tone letter in Budu in the Democratic Republic of the Congo, in Sabaot in Kenya, in some Grebo in Liberia, and in Papua New Guinea: Erima, Gizra, Go꞉bosi, Gwahatike, Kaluli, Kamula, Kasua, Kuni-Boazi, and Zimakani. The Unicode character used for the tone letter U+A789 ꞉ MODIFIER LETTER COLON is different from the punctuation (U+003A), as well from IPA's triangular colon U+02D0.

Mathematics and logic

The colon is used in mathematics, cartography, model building and other fields to denote 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". Unicode provides a distinct character U+2236 ∶ RATIO for mathematical usage.

The notation |G:H| may also denote the index of a subgroup.

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

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 (which is the ISO 31-11 standard), to mean “such that”. Example:

$S=\{x\in \mathbb {R} :1<x<3\}$ (S is the set of all x in $\mathbb {R}$ (the real numbers) such that x is strictly greater than 1 and strictly smaller than 3)

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 $\in$ symbol. Example:

$\lambda x\cdot x\ :\ A\to A$

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.

Computing

See also: Semicolon § Computing usage

In computing, the colon character is represented by ASCII code 58, (HTML &#58;) and is located at Unicode code-point U+003A (colon). Scripts comprising wide characters, such as kanji, use a full-width equivalent, located at Unicode code point U+FF1A ： FULLWIDTH COLON.

Several programming languages use the colon for various purposes.

A number of programming languages, most notably Pascal, and Ada use 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.

Labels – targets for jumps, notably goto, but also some switch statements – are general formed of a label name followed by a colon. These include C, and DOS batch files.

For the double colon used in computer programming, see the scope resolution operator, and class member access of C++.

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

In a number of languages, including JavaScript and Python, colons are used to define name-value pairs in a dictionary or object.

var obj = {
    'name': 'Charles',
    'age': 18,
}

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.

Addresses

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 (like http), and separates a port number from the hostname or IP address.

Other languages

In BASIC it's used as a separator between the statements or instructions in a single line, which is represented in other languages via the semicolon.

In Forth, 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:

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

while a double colon :: is read as "has type of" (confer scope resolution operator):

("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.

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.

APL uses the colon

• 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.

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 usage

On the Internet, a colon, or multiple colons, is sometimes used to denote an action or to emote, similarly to asterisks. In this use 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.

External links

• Colon - Definition and examples
• How to use the colon
• [1] How to use colons and semi-colons