Place a comma before the conjunction that joins two or more independent clauses together:
Do you want pancakes, or would you prefer waffles?
She loved his big muscles, and he loved her big bank accounts.
Greg drove to Spec’s, but they were out of drambuie, so he came back home.
Conjunctions that help join independent clauses together are called coordinating conjunctions. English has seven coordinating conjunctions:
*To remember the coordinating conjunctions, think F A N B O Y S.
A comma by itself cannot join independent clauses together. Each of the following examples are, therefore, a common variety of the run-on sentence—the comma splice:
Do you want pancakes, would you prefer waffles?
She loved his big muscles, he loved her big bank accounts.
Greg went to Spec’s, they were out of drambuie, he came back home.
See: Run-on Sentences