When to Use a Semicolon (and When Not To)

Job one: joining two sentences

A semicolon can sit between two independent clauses — groups of words that could each stand alone as a sentence — when the ideas are closely linked.

• The shop was closed; we went home.
• She rarely writes long emails; she prefers a quick call.

Each half is a complete sentence. You could use a full stop instead, and the grammar would still be correct. The semicolon tells the reader the two thoughts belong together, more tightly than two separate sentences would suggest.

The test: cover everything to the left of the semicolon. Is what remains a sentence? Now cover the right side. Is the left a sentence? If both answers are yes, a semicolon works.

With words like however and therefore

When the second sentence opens with a linking word such as however, therefore, meanwhile, otherwise or consequently, the semicolon goes before it and a comma usually follows it.

• The data was incomplete; however, the trend was clear.
• We missed the deadline; therefore, the launch moved to May.

A comma alone in these sentences — The data was incomplete, however the trend was clear — is a comma splice, one of the most common punctuation errors in formal writing.

Job two: lists that already have commas

When list items contain their own commas, semicolons separate the items so the reader can tell where each one ends.

• The tour stops in Paris, France; Rome, Italy; and Madrid, Spain.
• The panel included Dr Lee, a chemist; Ms Osei, an engineer; and Mr Hart, a lawyer.

With commas alone, the second example could be read as a list of six people.

When not to use one

• Before a list. That is a colon’s job: You will need three things: a pen, paper and patience.
• Before a fragment. We finally left; after three hours of waiting. The right side is not a sentence, so use a comma.
• With and, but or so. When two sentences are joined by one of these, a comma is normal: The shop was closed, so we went home.

Should you use them at all?

Semicolons are optional. Every semicolon between two sentences can be replaced by a full stop, and in web writing that is often the better choice, because shorter sentences are easier to scan. In essays and longer prose, an occasional semicolon shows the reader how two ideas connect. One or two a page is plenty.