By Mignon Fogarty Grammar Girl
Semicolons Separate Clauses
Here’s an example:
I have a big test tomorrow; I can’t go out tonight.
The two clauses in that sentence are separated by a semicolon and could be sentences on their own if you put a period between them instead:
I have a big test tomorrow. I can’t go out tonight.
Semicolons Create Variety
One reason you might choose to use a semicolon instead of a period is if you wanted to add variety to your sentence structure; for example, you might use a semicolon if you thought you had too many short, choppy sentences in a row. See More . . .