A colon is a mark consisting of two points arranged vertically (:). As a punctuation mark it is used in many languages. It is also used in mathematics to show ratios: a ratio of five to four can be written 5:4.
Like a full-stop (British English, known as a period in American English), a colon can follow a grammatically complete utterance. The same is true of the semi-colon. (In the case of the full stop/period the complete phrase is called a sentence.)
Unlike the full-stop and semi-colon, which merely mark an ending, a colon points forward, 'delivering' what is promised before it:
- I have something to tell you: you were right after all.
Colons are usually taught to children as introducing lists, and this is a typical function:
- Don't forget:
The colon is not usually followed by a capital (uppercase) letter without a line-break. The exception is where it is introducing a quotation where inverted commas are not used, the capital letter obviating the need for them, as in a headline:
- Smith: The situation is no longer sustainable.
- HAMLET: To be or not to be, that is the question.
A variant of this is when a colon introduces what could be called reported speech:
- Chico's explanation: Chico never wrote an autobiography, but his daughter Maxine...
The colon can change meaning. Consider:
- Manchester United fans look at the same players who were champions a few months ago, and conclude: Ferguson was the reason we won.
The colon here shows that the thoughts of the United fans are being imagined and "quoted". Without the colon, "and conclude Ferguson was the reason we won", it sounds as if the writer himself is a United fan.
As a punctuation mark, one increasingly sees the colon used with a space on either side, as there exist options for this to occur automatically on some processors (and it is a habit in texting). But in most languages, although not in French, this is not considered correct, and the colon should be used without a space before, exactly like other punctuation marks, the informal use of a hyphen as a dash - like this - being the only exception.
The spacing option is intended to be used in such examples as:
- pound : euro = 1.1311
For in this case the colon is not used as a punctuation mark, but as a mathematical symbol for a binary operation (such as the plus sign "+") to indicate proportion (2 : 4 = 1 : 2) or division (or ratio: 2 : 4 = 0.5).