Serial commas, I have learned, are a STYLE ISSUE, which means not everyone agrees on their usage. They are standard in American English but are used less often in British English. Some countries don't use them at all. But I'm going to address serial commas anyway because when my first novel came back from the editor at Penguin, she had added - oh, I don't know - I'll say hundreds of serial commas to my writing.
Kind of funny. I understand the reason for the term, since one the meanings of the word "serial" is: "of, relating to, or resembling a series" (www.dictionary.reference.com), and a "series" is "a group or a number of related or similar things, events, etc., arranged or occurring in temporal, spatial, or other order or succession; sequence." (www.dictionary.reference.com). The funny thing to me is that I can't help thinking "serial killer" every time I see that term.
I guess serial commas are kind of like serial killers in that they just can't stop.
When you are writing a sentence which includes a list of more than three items, you might be tempted to leave out the final one. It might feel redundant. But it's usually not, and it's often necessary to use one in order to avoid ambiguities.
Here are a few examples of when the serial comma.
I'm going to a movie with Cathy, a teacher and a librarian.
hmm. Is Cathy a teacher AND a librarian?
I'm going to a movie with Cathy, a teacher, and a librarian.
Ah. So you're going with three people, not just one. Or if it's two people, you could say "I'm going to a movie with Cathy, who is a teacher, and also with a librarian."
For dinner we had roast beef, salad, potatoes and ice cream.
Ew. Imagine eating potatoes and ice cream together?
For dinner we had roast beef, salad, potatoes, and ice cream.
Yum. Now I'm hungry.
I love to ski, play tennis, ride my bike and juggle.
Wow. You can bike and juggle simultaneously? You should join the circus.
I love to ski, play tennis, ride my bike, and juggle.
That makes more sense.
And one more just because it makes me laugh:
“With gratitude to my parents, Mother Teresa and the pope.” (from the Chicago Manual of Style)