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.

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 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 intending to be used in such examples as:

dollar : yen = 1 : 20