Two independent clauses are joined by a comma and together with a coordinating conjunction.

I told Mark that the alarm clock was rung, but I didn’t get up. As he was left with the alarm clock set, Mark woke up and called 911.

Do not put a comma before or after a coordinating conjunction that joins two verbs in the same

clause. Nelson is always complaining about not having any friends but is not a friendly person himself.

In other cases ‘but’ need not be separated by a

comma.