When a transitional expression—a conjunctive adverb or a transitional phrase—appears between independent clauses, it is preceded by a semicolon and usually followed by a comma.
When a transitional expression appears in the middle or at the end of the second independent clause, the semicolon goes between the clauses.
Transitional expressions should not be confused with coordinating conjunctions. Use a comma with a coordinating conjunction to link independent clauses.
Exercise: The semicolon and the comma 1
Exercise: The semicolon and the comma 2
Exercise: The colon, the semicolon, and the comma
Related topic:
Comma joining independent clauses
coordinating conjunction and, but, or, nor, for, so, yet; used to join elements of equal grammatical form.