Colons and semicolons are easy to confuse, and in my work as a proofreader and editor I see them misused daily. I’ll just go through the uses of each here.

Colon :

The colon’s main job is to join a statement with supporting facts. What comes after the colon cannot usually stand on its own – it needs its lead-up statement to justify it. For example:

There were three options: increase, decrease or remain the same.

Jackie had a brilliant plan: she would run away.

Colons can precede grammatically self-contained clauses or sentences, but that would be purely incidental (as in the second example).

Colons should always be used to introduce bullet lists and numbered lists if they are acting as through they are in a sentence. You could, for example, write “The bullet list below shows all the organisations involved.” That would not need a colon. However, if you’re using bullets to clarify a sentence with listed items separated by commas, you’d do something like this:

The countries involved were:

• Brazil;
• China;
• Egypt; and
• Australia.

Do not lead into a list with a semicolon. It’s incredibly common to see this done, but it’s wrong.

Semicolons make an appearance in that list, too. Although that’s how it should be presented in academic literature, in less formal bullet and number lists, there can be a bit of stylistic wiggle room.

Also worth noting is that items in bullet lists should not start with a capital letter unless they are proper nouns. They are not sentences; they are simply a way of making a list easier to understand. Again, capital letters feel a bit more natural to some people, and thanks to word processors auto-capitalising the start of every item, it’s very common to see. But in formal and academic contexts, treat bullet and numbered lists as a vertical sentences.

Semicolon ;

Semicolons are, as their appearance suggests, halfway between a colon and a comma. They are used to separate two independent but connected thoughts. The second statement can grammatically stand on its own, but without the first, it might not make much sense. Consider the following:

He opened the door and stepped inside. The echo of his footstep rang out.

He opened the door and stepped inside; the echo of his footstep rang out.

Both make perfect sense, but in the second example, it’s clearer that the echo and the step are connected. It feels like there’s continuity. There aren’t many situations where a semicolon is right but two sentences are wrong; it’s just a pleasant tool to deploy sparingly to let the reader recognise there’s a connection.

Comma splices

You can often use a semicolon to correct a comma splice. For example:

She toured the city, taking photo after photo, it was a lovely day.

which is wrong, can become:

She toured the city, taking photo after photo; it was a lovely day.

Capital letters after colons and semicolons

I mentioned above that both semicolons and colons should be followed by a lower-case letter unless it’s a proper noun. This is always true in British English. However, in the US, the rules are a little more flexible. Semicolons are usually followed by lower-case letters, but under some styles, the material after them is treated more like a new sentence, and therefore gets a capital. The important thing here is to be consistent with the style throughout.

The American way can have strange side-effects for the British reader, such as making mortals into deities: