They all mean the same thing: the comma that comes right before the and in a series. As in: Sean ate eggs, pancakes, and bacon for breakfast. That second comma is the one we’re talking about. For this post, I’ll refer to it as the serial comma (maybe it’s the connection to breakfast foods).
Many writers and editors feel strongly about whether the serial comma should be included. But here’s the thing: it’s not mandatory. The serial comma is a style issue. Chicago says to use it. AP says to nix it, except for a couple of situations. MLA uses it. So do APA and Gregg. Wired Style doesn’t.
Even if your house style doesn’t use the serial comma, it may make one or two exceptions, such as for clarity. Consider this sentence (frequently used to make this point):
I’d like to thank my parents, Ayn Rand and Jesus.
Is the speaker claiming an incredible heritage or thanking three groups? With a serial comma, we’d know for sure:
I’d like to thank my parents, Ayn Rand, and Jesus.
You could argue that the reader could quickly understand what the writer meant. And you’d be right, though the reader might have tripped at first. And that trip is something you want to avoid. That small moment might be enough to distract your reader from your message and have her slip away. Not good. The humble serial comma can help prevent that. Of course, so could a rewrite, but that’s not always ideal.
AP makes an exception for the serial comma when the final item in the series contains a conjunction (e.g., and, but, or):
Sean ate pancakes, juice, and ham and eggs for breakfast.
AP also makes an exception when the series is complex:
Sean considered whether to eat a large breakfast of eggs and pancakes, to have a light breakfast of hot oatmeal, or to skip breakfast completely.
Should you use the serial comma in your writing? I’m all for it. Consistently applying the serial comma is easier than not using it but watching for exceptions. And then asking the same of your editor. It’s too easy to miss one case that really should have the comma and cause your readers to stumble over your meaning. If you use a specific style guide, follow that.
Again, there are exceptions. I recently edited a marketing book for an author who followed Chicago but who consistently didn’t use the serial comma in his text. Clearly, his own rule was not to use the serial comma. Did it make sense for me to insert another comma in every series? Certainly not! I kept an eye out for any exceptions and let it go. It was the author’s choice, not mine. If I had been working for a publisher instead of the writer, I would have followed what the publishing house dictated. Some will have a style ruling on the serial comma; others will have a rule that says the writer gets to choose.
In the end, the serial comma is a style rule. And in that case, he who pays the bills gets to make the rule.
For more on the comma, check out my post on the direct address comma.