Add chopped potatoes, mushrooms, and herbs, and cook for 20 minutes.
Is a comma necessary before the second "and"? And if it is necessary, why? Please help find a grammar rule for this case.
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
A comma is necessary before the second "and". This is because it is being used as a conjunction to join two independent clauses ("Add chopped potatoes, mushrooms, and herbs", and "Cook for 20 minutes") into a single compound sentence. When "and" is used in this way, we must mark the separation between clauses with a comma.
To help illuminate the discussion, some further relevant context is that a comma is not necessary before the first "and," because this is the optional comma placement known as the "Oxford comma" -- it is highly controversial in terms of taste (i.e. you'll very rarely find one other who both chooses to use the Oxford comma in some cases, yet omits it in others) but in terms of correctness, both ways are traditionally accepted in most contexts and according to most official styles.