The Oxford comma is the comma placed after the second-to-last item in a list of three or more, just before and or or:
- With it: We need flour, sugar, and eggs.
- Without it: We need flour, sugar and eggs.
Both sentences are correct English. The comma takes its name from Oxford University Press, whose house style has long used it. It is also called the serial comma, and sometimes the Harvard comma.
Who uses it and who does not
Style guides disagree, which is why the question never goes away:
- Use it: The Chicago Manual of Style, APA style, MLA style and Oxford University Press's own guide, New Hart's Rules. Much American academic and book publishing follows them.
- Leave it out: the Associated Press Stylebook, which most American newspapers follow, uses it only where a sentence would otherwise be unclear. Many British newspapers and magazines also leave it out.
If you write for a publication, a university or a company with a style guide, follow the guide. If nobody has decided for you, either choice is fine.
When the comma changes the meaning
The argument for the Oxford comma is that it removes a misreading. Compare:
- I dedicate this book to my parents, Anna and Tom. Are Anna and Tom the parents, or are they two more people?
- I dedicate this book to my parents, Anna, and Tom. Now it is clearly four people.
The comma is not a cure for every list, though. With one parent the same comma creates the ambiguity instead: to my mother, Anna, and Tom could mean that Anna is the mother. When a list can be read two ways whatever you do with the commas, the fix is to reorder or rewrite it: to Tom, Anna and my mother.
Lists that need more than a comma
- Two items take no comma at all: salt and pepper, not salt, and pepper.
- Items that contain commas are separated with semicolons: We visited Lyon, France; Turin, Italy; and Geneva, Switzerland. Here even writers who skip the Oxford comma usually keep the final semicolon.
- Lists joined with or follow the same rule as lists joined with and: tea, coffee, or juice.
Consistency is the real rule
A reader rarely notices which style you chose. They do notice a document that uses the comma in one list and drops it in the next, because it looks like a mistake even where neither version is one. Pick one style for a document and keep to it, the same way you would pick one spelling variety (see American and British Spelling).
How the spell checker helps
Because both styles are correct, no checker can choose for you. What the Spell Checker's punctuation rules do look for are the comma slips that are mistakes in any style: a stray space before a comma, a missing space after one, or two commas in a row. A final read with your own style in mind takes care of the rest; What Is Proofreading? has a short routine for it.