Subscribe to The Chicago Manual of Style Online
Q. When I began learning English grammar from the nuns in or about 1951, I was taught to NEVER use a comma either after or before independent clauses or compound sentences. Did the rules of English grammar and punctuation change while I was in that three-week coma in 1965 or in the years that it took to regain my basic and intellectual functioning before I returned to teaching?
A. I’m sorry I can’t account for your state of mind, but standard punctuation calls for a comma before a conjunction that joins two independent clauses unless the clauses are very short. Please see CMOS 6.22. I would go further and suggest that it’s a good idea to reexamine any rule you were taught that includes the word “never” or “always.”