2.2. Commas

2.2.1. In compound sentences

Use a comma to join clauses in a compound sentence, unless the clauses are short and have a similar theme:

I spent five hours working on this document, but I lost it when my computer crashed.
Do you want to go the mall and the grocery store with me, or are you going to watch football instead?
You wash and I'll dry.

A comma can be omitted from a sentence with several clauses, but only when there's little chance that the sentence could be misread without it:

We played football all afternoon and were completely exhausted but we still stayed up watching movies all night.

That sentence is fine, but adding a comma before "...but we still stayed up..." would provide a pause and avoid the chance of having it read like a run-on sentence.

In a compound sentence containing several short independent clauses, separate the clauses with commas AND USE A COMMA BEFORE THE CONJUNCTION:

You need to go to the grocery store for milk, drop off my dry cleaning, and pick up your little sister from soccer practice.