When I was learning grammar, the rule concerning commas and lists was easy. In any list, and a list was defined as having three or more items, you put a comma between each item, including before the “and”. In other words, your list liked like this: “I need to go to the store to buy milk, eggs, butter, and cheese.”
When my daughter went to school, they taught her to omit the comma before the “and”, making the list look like this: “I need to go to the store to buy milk, eggs, butter and cheese.” This works well, unless the last two items on the list could be mistaken for a single item. For example, what if my list was slightly different? “I need to go to the store to buy milk, eggs, butter, macaroni and cheese.” Do I need to buy both cheese from the dairy case and macaroni from the pasta aisle, or am I looking for a box of macaroni and cheese from the prepared foods aisle? My daughter’s teacher told me that the comma belonged before the “and” only if there was some ambiguity in the list.
Out of habit, I continued to always put the comma before the “and”. None of my college teachers have objected to its presence. When I sat down to write this post, I decided to do a little research and find out the current rule of correct comma usage. I searched at least a dozen sources, and found a great deal of confusion. No one seems to agree whether to put the comma in before the “and”, or not. Some sources said the comma should absolutely be there, while others said it should only be included in cases of ambiguity. There didn’t seem to be a clear difference between US and UK rules. Finally, I found a resource from the University of North Carolina which seems to be the final authority. They say, “Put commas between items in a list. When giving a short and simple list of things in a sentence, the last comma (right before the conjunction–usually “and” or “or”) is optional, but it is never wrong. If the items in the list are longer and more complicated, you should always place a final comma before the conjunction.”
I’ll keep adding that final comma out of habit, but it’s good to know that while it is optional, it is never wrong.