The colon is a useful punctuation tool which should only be used in certain specific circumstances. It usually introduces an explanation, definition, description or list of items relating to the clause immediately prior to the colon. The information that comes after a colon illustrates or provides specific examples relevant to the more general statement which comes before a colon.
The colon was once memorably used in a classic grammar joke by the comedy group The Mighty Boosh, when Howard tells Vince about his future career plans. ‘Howard Moon, colon, explorer’, he proclaims. ‘Colon explorer?’ says Vince, ‘I don’t think that’s got a good ring to it’. It isn’t quite as amusing when written down, but it does demonstrate how the colon is used to introduce a description in a sentence:
‘Howard Moon: Explorer’.The clause or list which comes after a colon does not have to be a complete sentence in its own right but the clause preceding the colon most definitely should! It would therefore be incorrect to write:
‘Ingredients for pancakes: flour, eggs and milk’but correct to write:
‘The ingredients needed to make pancakes are: flour, eggs and milk’.
The first clause of the first sentence does not contain a verb and so does not make grammatical sense, whilst the second does and so is correct.
A colon is also used in several other instances:
-when writing ratios, ‘2:3’
-when introducing speech in a play, ‘Macbeth: Is this a dagger which I see before me?’