When reading, I’ve noticed that most people leave out the final comma when separating three or more words in a series. In looking up the rule online, I have found that the final comma is always used. There is an interesting article on the subject that can be found online.
When you take a college proofreading class, you will find that the comma in a series includes the word before the conjunction. The Gregg Reference Manual, used in one such college proofreading class, states:
When two or more items are listed in a series and the last item is preceded by and, or, or nor, place a comma before the conjunction as well as between the other items.
Many people, however, leave this comma out. This may be because many of them have been taught that way. Some may state the rule that newspapers allow their writers to omit the final comma. Why is it allowed in newspapers? Simply put, this is allowed in newspapers to save space.
Experts don’t agree about this. To be safe, though, it’s best to include it when writing more than two words in a series. However, the missing serial comma is necessary to resolve doubt. Let’s look at the following:
Michelle left California with her sisters, Mary and Jane.
Michelle left California with her sisters, Mary, and Jane.
The second sentence takes on a whole new meaning. Is Mary and Jane her sisters, which the first sentence might suggest? Or did Michelle leave with her sisters and two friends.
Whether we use the final comma or not is up to us. If we leave it out, it may easily go unnoticed, as many omit the final comma. However, we need to make sure that we use it when it might create confusion about our subject.