Strunk & White provide a reliable authority on many questions of style and grammar. If you ever arrange words for others to read, it's worth having a copy of their little book to hand. In one case, though, they're wrong:

2. In a series of three or more terms with a single conjunction, use a comma after each term except the last.

Strunk, W Jr & White, EB The Elements of Style (4th Ed.)

The Oxford comma — please don't.

What is the Oxford comma?

Sometimes called the serial comma, the Oxford comma may appear before the ‘and’ that precedes the last item in a list:

I went to the shop to buy apples, pears, and oranges.

It's the comma between ‘pears’ and ‘and’.

Why not?

It's ugly, and it's very rarely necessary. Do use it when you need to - the sense of what you're trying to say is the most important thing - but make sure it really is necessary. For example:

I went to the shop to buy bread, steak and kidney pie and milk.

‘Steak and kidney pie’ is a single item that introduces an extra ‘and’, causing the reader to pause for a moment to figure out what you're trying to say. This time, the Oxford comma makes the reader's life easier:

I went to the shop to buy bread, steak and kidney pie, and milk.

Alternatively, you could try reordering the list. It doesn't always work, but try it to see:

I went to the shop to buy bread, milk and steak and kidney pie.

Does it matter?

Really? No.

Like almost anything to do with language, the Oxford comma sits between convention and taste.