The English word "colon" is from Latin: colon (pl. cola), itself from Ancient Greekκῶλον (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 Greeksemicolon.) 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 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 capitalise 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
Daequan was so hungry that he ate everything in the house: chips, cold pizza, pretzels and dip, hot dogs, peanut butter, and candy.
Colon used before a description
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.
Colon before definition
For years while I was reading Shakespeare's Othello and criticism on it, I had to constantly look up the word "egregious" since the villain uses that word: outstandingly bad or shocking.
Colon before explanation
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:
Dinner: chips and juice. What a well-rounded diet I have.
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.
An appositive colon also separates the subtitle of a work from its principal title. (In effect, the example given above illustrates an appositive use of the colon as an abbreviation for the conjunction 'because'.) 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:
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 capitalsEdit
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.
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.
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.
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 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.
U+02D0ːMODIFIER LETTER TRIANGULAR COLON, used in IPA.
U+02D1ˑMODIFIER LETTER HALF TRIANGULAR COLON, used in IPA.
U+FE55﹕SMALL COLON, compatibility character for the Chinese National Standard CNS 11643.
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 and Java 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.: 131  In a related use, Python uses a colon to separate a control statement from the block of statements it controls:
iftest(x):print("test(x) is true!")else:print("test(x) is not true...")
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.
The ML languages (such as Standard ML) 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.: 20, 70
MATLAB uses the colon as a binary operator that generates vectors, as well as to select particular portions of existing matrices.
to introduce a control structure element. In this usage it must be the first non-blank character of the line.: 64
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).: 64
to separate a guard (boolean expression) from its expression in a dynamic function.: 111 Two colons are used for an Error guard (one or more error numbers).: 115
Colon + space are used in class definitions to indicate inheritance.: 135
⍠ (a colon in a box) is used by APL for its variant operator.: 340
The colon is also used in many operating systems commands.
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.
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.
:Dented text by the means of a colon.
::The gap increases with colon number.
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)[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.
Colons can also be used to represent eyes in emoticons.
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