1. Join two related complete sentences
Each side must be able to stand alone as a sentence: The train was late; we missed the opening. If either side is a fragment, the semicolon is wrong.
2. Before a transition word joining two sentences
Words like however, therefore, otherwise, meanwhile and as a result do not join sentences on their own. Put a semicolon before them and a comma after: It rained; however, the match went ahead.
3. Separate list items that already contain commas
We visited Lyon, France; Porto, Portugal; and Split, Croatia. Here the semicolons act as "super-commas" so the reader can see where each item ends.
When not to use one
Not before a list (use a colon), not before "and", "but" or "so" when joining two sentences (a comma does that), and not between a sentence and a fragment.
Frequently asked
Do you capitalise after a semicolon?
No, unless the next word is a proper noun or "I".
Can I use a semicolon instead of a comma?
Only where the comma is joining two full sentences (a comma splice) or separating items that contain commas.
Is a semicolon the same as a colon?
No. A colon introduces something (a list, an explanation, a quotation); a semicolon links two equal, closely related sentences. See the colon vs semicolon guide.