Colon punctuation: sounds painful, but it’s actually a lot of fun.
It could be argued that the colon is the oldest of all our punctuation marks. It was first coined as a term by the ancient Greeks, although it meant a passage of text or a complete thought rather than the mark that they began to add to the text to distinguish the section. The symbol itself was a dot in the middle of the line (∙).
Texts were always to be read aloud, so the ∙ represented a pause for breath, a change of idea perhaps. A double dot (:) was then introduced to show a change of speaker or to act as a full stop.
As English became a printed language, the colon was introduced into texts in Britain around 1600, now acting as a midway step between the pausing comma and terminal full stop.
Its functions in English now are generally restricted to preceding an explanation or list, to indicate a subtitle (The Grammarian II: Colon Warrior), or show a ratio (15:1). Americans also use it when reporting the time (11:25) and are probably more familiar with its use in Biblical citations (John 3:16).
Historically, in British English, it was frequently followed by a dash (:–), which was used to highlight the pause that was expected afterwards. This combination was known to typographers as ‘the dog’s boll*cks’; it seems a shame that both the symbol and its description have fallen out of fashion.
Using a colon with clauses
The most common usage of the colon, and perhaps the trickiest to get a solid understanding of, is when it shows that the following clause will shed some light on the preceding sentence, which should be a complete sentence.
It can be followed by a list
The shopping list was quite exotic: mangos, macadamia nuts, parrot food, and cockroach spray.
It can be followed by a description
The sky over Headingley was like Tupperware: thick, white-grey and somehow claustrophobic.
It can be followed by an explanation
It was all my fault: I drank too much whiskey and should never have attempted the karaoke.
It can be followed by a definition
There’s only one word for that: magic darts.
Colons are also used to introduce speech or an unmarked quotation, usually in play scripts (Narrator: I see what you’ve done there).
Rules of the colon
As any colorectal oncologist can tell you, the rules of the colon are often neglected or poorly understood.
Generally, in British English, the clause that follows is not capitalised (Here is the weather: rainy with showers), but in America, the first letter is normally a capital (Move to San Diego: It never rains).
Writers will often use a colon to head up a list, but the preceding sentence should always be a complete one.
For my birthday, I want: a jetpack, a flying car and a time machine, for example, is incorrect. However, my birthday needs are simple: a pint of foamy ale, a handwritten card and the company of one or two dear friends is fine.