We consider the habit of using the Oxford Comma – so-called because it has shown up as required editorial practice in the Oxford University Press style manual for over a century – to be non-negotiable. We hope you do, too, because the Oxford Comma is a central principle of clear writing and effective editing.

What is the Oxford Comma? It’s the final comma you place before the conjunction in any list of three or more items. So, for instance:

When we go to the beach, we should bring the picnic basket, the suntan lotion, and the blanket.

The word “and” is the conjunction in that sentence. Notice that the Oxford Comma comes right before it. Consider the alternative:

When we go to the beach, we should bring the picnic basket, the suntan lotion and the blanket.

While some writers and editors argue that this sentence is more direct and less cluttered than the first, and is therefore preferable, we disagree. The lack of a comma separating the final two elements suggests some kind of special connection between the suntan lotion and the blanket,  a connection that doesn’t exist. Failing to insert the comma adds ambiguity, and removing ambiguity is what clear writing and effective editing is all about.

A minor quibble? Not really. Suppose you’re drafting a book dedication, and you write:

“I dedicate this book to my parents, Emily Dickinson, and God.” 

Strange, perhaps, but at least the message is comprehensible. Notice how the meaning changes, though, if you omit the final comma: 

I dedicate this book to my parents, Emily Dickinson and God.

No doubt about it. The meaning changed, and not in a good (or intentional) way! Good writing is all about clarity. If you’ve got a list of three or more items, follow the Oxford University style manual. Place a comma before the preposition.