I have a question about commas used in lists.
I learned in school, back when we were still using stone tablets, the comma was not placed before the and, but or or.
However, I then read then learned the comma was to be used between the penultimate and ultimate things.
So I go to the store and buy apples, bananas, grapes, and watermelons now.
Except maybe I'm to go to the store and buy cereal, bread, potato chips and pretzels.
Does it matter? Is it geographically determined? Is the rule just a matter of being consistent?
Does that last comma belong there or not?