Example: I went walking with my dogs, John, and Bob.
That comma after John is the Oxford comma. Some people think it’s unnecessary, but look what happens to the sentence if it’s taken out.
Example: I went walking with my dogs, John and Bob.
In the first example I am walking with my dogs and two people named John and Bob. In the second, my dogs’ names are John and Bob.
The Oxford comma is primarily used in America, while in England it’s not used. It’s also not used in journalism. Prominent style guides like Strunk & White’s Elements of Style use it.
I’ve always been taught that a series of items needs the comma before the and, so I use it. I also teach it. This year is the first year I’ve had students question me on the Oxford comma. Oh, they didn’t know what it was called, and they didn’t know it by name, but they’d been taught it isn’t necessary. I told them, with gritted teeth, that I would accept the punctuation either way. This is hard for me since I am a punctuation perfectionist. I see their sentences without that comma and want to go all red ink on them.
Now the school year is ending and I feel it is my duty to fully explain this controversy. My students are used to punctuation being black and white: Capitalize the first letter in a sentence; put a period at the end; comma in a compound sentence. For the first time, they will be given a choice about a comma. I won’t tell them it’s wrong to use the Oxford comma, but I will give them the ultimate directive: Put a comma if it is needed for clarity. If the sentence makes people wonder what you mean, like in my example, put the comma. If you don’t want to think about whether your comma is needed or not, put the comma. If you want to get an A+ in grammar always (in America), put the comma. If you go to college in England, don’t put the comma. There. Grammar clear as mud. You’re welcome.