What is the Oxford comma and does it matter?

The Oxford comma — also called the serial comma — is the one placed before and in a list of three or more items: red, white, and blue, rather than red, white and blue.

It matters because it sometimes resolves an ambiguity that nothing else will.

The standard demonstration: I'd like to thank my parents, Ayn Rand and God. Without the serial comma this can be read as naming the parents. With it — my parents, Ayn Rand, and God — the list is unmistakably three separate items.

The reverse case exists too, which is why the comma is a tool rather than a rule. We interviewed the director, a novelist, and a critic implies three people. Without the comma, the director, a novelist and a critic could describe the director as being both. Adding the comma can also create an appositive reading it did not have. The honest position is that either style can be ambiguous in specific sentences, and the writer's job is to notice.

Which to use is largely a house-style decision. Most American style guides require it, including Chicago and the Oxford style it is named after. Most British journalism omits it, as do the Associated Press and many news outlets, chiefly to save space.

The only genuinely wrong choice is inconsistency within one document.

It has occasionally had legal consequences. A 2017 case in Maine turned on a missing serial comma in an overtime statute, and the ambiguity cost the employer several million dollars in settlement.

When a list is ambiguous either way, rewrite it rather than trusting punctuation to carry the meaning.