When related independent clauses appear in one sentence, they are ordinarily linked with a comma and a coordinating conjunction (and, but, or, nor, for, so, yet). The coordinating conjunction signals the relation between the clauses.
If the two independent clauses are closely related and the relation is clear without the conjunction, the clauses may be linked with a semicolon instead.
A semicolon must be used whenever a coordinating conjunction has been omitted between independent clauses. To use merely a comma creates a kind of run-on sentence known as a comma splice.
Occasionally, a semicolon may be used to emphasize a sharp contrast or a firm distinction between clauses joined with a coordinating conjunction.
Exercise: The semicolon and the comma 1
Exercise: The semicolon and the comma 2
Exercise: The colon, the semicolon, and the comma
Related topic:
Run-on sentences
independent clause A word group containing a subject and a verb that can or does stand alone as a sentence.
run-on sentence An error in which a sentence contains two or more clauses that have not been joined correctly.