comma before a coordinating conjunction
The comma in a compound sentence.
When a coordinating conjunction—and, but, or, nor, for, so, yet—joins two independent clauses, put a comma before it: The train was late, but we waited. Do not use that comma when the second part has no subject of its own: The train was late but arrived intact.
Examples
UseThe brief was long, but the judge read it.
UseThe brief was long but clear.
AvoidThe brief was long but the judge read it.