I have always found difficult to understand whether comma is required before the last item in the series. I have also seen some books which indicate that the decision to add a comma should depend on whether there are just two or more items in the series.
But I apply the foll rules to decide whether a comma is required or not. Would be glad if someone can have a look at this and clarify
-- rules are slightly different if there are 2 or more than 2 items in the series
-- if the items are only 2 and it the sentence unambiguously indicates that there are more than 2 items then comma is not neccessary.
e.g. ABC is taking lessons in x and y.
What the author meant is that ABC is taking lessons in subject x and subject y. But the above sentence can mean ABC is taking lesson in a subject called x & Y (like maths and stats). So here a comma is neccessary
e.g. x and y subjects are very tough.
In this case, adjective makes it very clear that these are 2 different subjects. So no comma is neccessary
-- When the number of items are more than 2 then the above logic does not work. So the items have to be neccessarily separated using commas
e.g. x, y and z subjects are very tough.
In this case, without a comma after y it would be impossible to make out if y and z are one subjects are two separate subjects.
Do let me know if my way of comma usage is correct.
It's a very difficult issue, and one that people actually argue about.
A comma used to separate the last two items on a list is called "the Oxford comma", although it has nothing to do with the English city of Oxford. Some people say the Oxford comma is always required, other people say it's only required to avoid ambiguity.
And that's the problem: there is no definitive answer. If you get a job in publishing or journalism, you will probably have to follow a "style guide" which tells you what rules to follow for spelling and punctuation, including the use of the Oxford comma. Each publisher has its own style guide, and that, I'm afraid, is the best answer anyone can give you.