The colon, , is a punctuation mark consisting of two equally sized dots aligned vertically. A colon often precedes an explanation, a list, or a quoted sentence. It is also used between hours, minutes and seconds in time, between certain elements in medical journal citations, between chapter and verse in Bible citations, between two numbers in a ratio, and, in the US, for salutations in business letters and other formal letters.
History
In Ancient Greek, in rhetoric and prosody, the term (, 'limb, member of a body') did not refer to punctuation, but to a member or section of a complete thought or passage; see also Colon (rhetoric). From this usage, in palaeography, a colon is a clause or group of clauses written as a line in a manuscript.
In the 3rd century BC, Aristophanes of Byzantium is alleged to have devised a punctuation system, in which the end of such a was thought to occasion a medium-length breath, and was marked by a middot . In practice, evidence is scarce for its early usage, but it was revived later as the ano teleia, the modern Greek semicolon. Some writers also used a double dot symbol , that later came to be used as a full stop or to mark a change of speaker. (See also Punctuation in Ancient Greek.)
In 1589, in The Arte of English Poesie, the English term colon and the corresponding punctuation mark is attested:
In 1622, in Nicholas Okes' print of William Shakespeare's Othello, the typographical construction of a colon followed by a hyphen or dash to indicate a restful pause is attested. This construction, known as the dog's bollocks, was once common in British English, though this usage is now discouraged.
As late as the 18th century, John Mason related the appropriateness of a colon to the length of the pause taken when reading the text aloud, but silent reading eventually replaced this with other considerations.
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 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.
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.
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 was launched at 09:15:05.
British English and Australian English, however, more frequently use a point for this purpose: The programme will begin at 8.00 pm. You will need to arrive by 14.30.
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"
Appositive Luruns could not speak: he was drunk.Example quoted in An Educational Companion to Eats, Shoots & Leaves by Lynne Truss
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
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 British industry-standard templates for written performance dialogues, such as in a play. The colon indicates that the words following an character's name are spoken by that character.
Patient: Doctor, I feel like a pair of curtains. Doctor: Pull yourself together! The uniform visual pattern of <character_nametag : character_spoken_lines> placement on a script page assists an actor in scanning for the lines of their assigned character during rehearsal, especially if a script is undergoing rewrites between rehearsals.
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 (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 mark 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, and k:a for kyrka ("church") – for instance Svenska k:a (Svenska kyrkan), the Evangelical Lutheran national Church of Sweden. 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.Compare:
Word separator
In Ethiopia, both Amharic and Ge'ez script used and sometimes still use a colon-like mark as word separator.
Historically, a colon-like mark was used as a word separator in Old Turkic script.
End of sentence or verse In Armenian, a colon indicates the end of a sentence, similar to a Latin full stop or period.
In liturgical Hebrew, the sof pasuq is used in some writings such as prayer books to signal the end of a verse.
Score divider In German, Hebrew, and sometimes in English, a colon divides the scores of opponents in sports and games. A result of 149–0 would be written as 149 : 0 in German and in Hebrew.
Typographical conventions
Use of capitals Use of capitalization or lowercase after a colon varies. In British English, and in most Commonwealth countries, the word following the colon is in lowercase unless the colon is followed by several complete sentences or it would be normally capitalized for some other reason, as with proper nouns and acronyms. British English also capitalizes a new sentence introduced by a colon's segmental use.
American English 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 lowercase letter unless the upper case is required for other reasons, as with British English. German usage requires capitalization of independent clauses following a colon.Duden Newsletter vom 24 August 2001 Dutch further capitalizes the first word of any quotation following a colon, even if it is not a complete sentence on its own.
Spacing and parentheses
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.
In modern typography, a colon will be placed outside the closing parenthesis introducing a list. In very early English typography, it could be placed inside, as seen in Roger Williams' 1643 book about the Native American languages of New England.
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 (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:_____". For these uses, there is a dedicated Unicode symbol () that is preferred in some contexts. Compare (ratio colon) with 2:3 (U+003A ASCII colon).
In some languages (e.g. German, Russian, and French), the colon is the commonly used sign for division (instead of , which may mean subtraction or a range of values.).
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 (which is the ISO 31-11 standard), to mean "such that". Example: (S is the set of all in (the real numbers) such that is strictly greater than 1 and strictly smaller than 3)
In older literature on mathematical logic, it is used to indicate how expressions should be bracketed (see Glossary of Principia Mathematica).
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).
Computing