The semicolon is stronger than a comma but weaker than a period. However, it can assume either role, although its function is usually closer to that of a period. Semicolons should be used sparingly since they often cause sentences to be long and cumbersome.
The most common use of a semicolon is between two independent clauses that are not joined by a conjunction. Example: The road was long; the road was dangerous.
A semicolon should precede adverbs such as then, however, thus, hence, indeed, besides, and therefore when it is use transitionally between two independent clauses.
Example: Richard is going to London; however, he doesn’t have his reservations yet.
When a semicolon is called for at the end of material enclosed in parentheses or brackets, the semicolon should follow the closing parenthesis or bracket. Example: She placed her manuscript in an envelope (her final draft); then, with relief, she sealed the envelope.