I've been taught that when making a list of three or more items, you use a comma before the "and" in the list.
"He had apples, oranges, and pears."
If you leave out the comma, it's supposed to mean that the last two items are a unit.
"He had a door, a doorknob, and a lock and key."
(Even though there, I guess you're still using the comma before the first "and.")
The other day, though, my English teacher told me that you're supposed to leave out that last comma.
"He had apples, oranges and pears."
"He had a door, a doorknob and a lock and key."
Those last two look wrong to me, but, which is it?
EDIT: And if there's a difference between American English and British English in this, I'd like the American English one, please.
Thanks!