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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.