This week old school grammarian Mr Gwynne gives the low-down on colons and semicolons, the source of much grief to the under-educated.

The colon introduces (a) direct speech in inverted commas, (b) a quotation, or (c) a list of examples or particulars (as in ‘I am learning the following: English, Latin, and Greek’). The comma is now very often – perhaps more often than not – used in front of a quoted sentence in place of the colon. I am against this, as is the Economist‘s best-selling manual, Style Guide.

The semicolon, indicating a longer pause than does the comma, is used

(a) when two or more sentences are thrown into one, for reasons either of sound or of sense (as in ‘I think you are wrong; you may be right’);

(b) to separate rather lengthy main clauses;

(c) to separate subsidiary clauses which, in succession, depend on the same main verb (as in the very sentence you are reading now, in which all the subsidiary verbs depend on the verb ‘is used’, immediately before the (a), (b), (c) … lettering starts);

(d) to divide clauses which are connected by a conjunction which either

i) indicates an alternative (as in ‘Pay attention; otherwise you won’t learn anything’) or
ii) indicates an inference (as in ‘I am hoping that you are paying attention; for this information about punctuation is really important’).

From Gwynne’s Grammar (Ebury Press)