The English language has only 14 punctuation marks. Of these, I find the two that cause the most issues for writers are the colon and the semicolon.
A good number of rules govern usage of colons and semicolons. For full details, I recommend checking out the Oxford Style Guide and/or the Oxford A–Z of Grammar and Punctuation.
In simple terms, a colon (:) is an introducer. It points forward and introduces:
- A list (as above).
- An extended quotation or direct speech. Mr Jones says: ‘I’m delighted by this promotion…’
- An explanation or amplification of the preceding part of the sentence. There was only one thing to do: run.
The latter usage is often the trickiest to master. Basically, a colon is a rather theatrical punctuation mark: it makes the reader pause and announces something to come which will add new information to the part of the sentence before the colon (as in this sentence).
Often, the part of the sentence before the colon is a complete sentence in itself, and the colon could be replaced with words like namely, that is, for example, for instance, because or therefore.
The semicolon (;) is a little like a comma but with special powers. It has two main jobs:
- It can join two separate sentences that are closely related. It was his first job as a salesman; before this, he had been a teacher.
- It can divide up lists that contain commas to avoid confusion. I ordered a prawn cocktail, not with salad; a steak, chips and peas; an ice cream sundae without nuts; and a pitcher of beer.
Some authors sprinkle semicolons liberally through their book, using them to join a word or phrase to a main clause. For example:
He painted the door; blue, to match his eyes.
Here, you need a colon or, if you prefer a less formal feel, a dash.
Colon or semicolon?
Here’s an example of choosing between a colon and a semicolon:
The chancellor nodded; they had discussed this before.
You want to connect these two sentences together, to show that the chancellor is nodding because he understands and he understands because they have discussed this before.
You can’t use a comma, because this would be a comma splice (i.e. two independent clauses incorrectly joined by a comma to make one sentence). You could use a colon here, but it’s not ideal, because the punctuation after nodded isn’t meant to create a fanfare or signal to the reader, ‘Ah-ha, now I’m really going to explain the phrase “The chancellor nodded”.’ So you choose a semicolon, which works perfectly, joining two sentences that are closely related.
The usual style in British English is to use lowercase on the first word after a colon, so:
I gave him a go: it was his right, after all.
In American English, the first word after the colon may be capitalised if it begins a complete sentence:
I gave him a go: It was his right, after all.
In a heading, you usually capitalise the word after the colon:
Baking cookies: A beginner’s guide
You can also capitalise after a key word or phrase:
Tip: Check your tyres before a cycle ride.
You never need to capitalise after a semicolon.