I recently received a query from a friend and client asking me to explain the uses of semicolons and colons:
Semi-colons are used to join two complete sentences that have a closely related meaning or are a logical follow-on from one another. They can also join two sentences which have contrasting meaning, to accentuate the contrast. You could also use conjunctions to join these sentences, but sometimes they have more impact if you just use a semicolon and leave out the conjunction.
Mary tells lies, so Jane can’t stand her.
Mary tells lies; Jane can’t stand her.
Mary lies; no one believes her.
Mary lies; Jane tells the truth.
Semicolons are also used in sentences with a number of ideas listed in sub-clauses, to make it clearer which sub-clause belongs to which idea.
We needed to buy food for the trip: eggs for breakfast, but not brown eggs; sausages for the barbecue, but not spicy sausages; vegetables; fruit, which had to be plump and firm, according to Ruth; and some plain rolls.
Colons are also used to precede lists, but should only be used after a complete sentence, so you wouldn’t use one after ‘is’ in ‘The question is, what is your easy…’
You’d also use a colon before a quote, though you don’t have to, and in plays to show who is going to speak next.
You can also use a colon to join two complete sentences. If the second sentence explains the first sentence, then a colon can be appropriate.
Tommy has a problem: he can’t sleep without his blue blankie.
Colons can also be used to join a single sentence to an explanation:
Jane has one problem: Mary.
There are times when these uses can overlap so that both a semicolon and a colon would be correct, but they would lend different emphasis to the different parts of the sentence.
Mary lies: no one believes her.
Using a colon here tells the reader that no one believes Mary because she is known to be a liar. It’s a direct result of Mary’s own actions.