Comma Between Independent Clauses Joined by a Coordinating Conjunction
The Comma
Rulebase Summary
Use a comma to separate independent clauses in a compound sentence when they are joined by a coordinating conjunction (and, but, or, nor).
If both clauses are short (roughly five words or fewer), the comma may be omitted. When a dependent clause at the beginning of the sentence equally governs both independent clauses joined by 'and' or 'or,' do not use a comma between them.
However, if the conjunction is 'but,' the comma may still be needed because 'but' often signals that the dependent clause no longer governs equally.
Examples
A. I locked the front door and I set the alarm before leaving.
A. I locked the front door, and I set the alarm before leaving.
Comma before 'and' joining two independent clauses.
A. She reviewed the contract but she did not sign it that day.
A. She reviewed the contract, but she did not sign it that day.
Comma before 'but' separating independent clauses.
Transcript Notes
This comma depends on having an independent subject and verb after the coordinate conjunction. The subject after the conjunction can be included or omitted without affecting the meaning, but it changes the structure. If the subject and verb are there, use a comma; if only the verb is there, do not use the comma.
Look beyond what might be right after the conjunction to find the independent subject and verb -- the comma depends solely on that subject and verb.
The core principle: use a comma before a coordinate conjunction when there is an independent subject and verb after it, without regard to what else may be going on in the sentence. This applies even when the sentence before the conjunction is a fragment (common in witness answers), when the speech has a stumble after the conjunction, and even when the independent clause after the conjunction is interrupted and not completed.
Style Guide References (3)
Morson's
BGGP
BGGP