Colon (punctuation)

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.

Punctuation mark

Like a full-stop (BrE, known as a period in AmE), 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: not usually. 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.

Or play:

HAMLET: To be or not to be, that is the question.

Spacing

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

But in this case the colon is not used as a punctuation mark, but as a mathematical symbol for a binary relation (proportion: 2 : 4 = 1 : 2) or a binary operation (division or ratio: 2 : 4 = 0.5).