What is the Oxford comma?

Also known as a serial comma, the Oxford comma derives its common name from its use in by the Oxford University Press (OUP).

The OUP's style guide states: "In a list of three or more items, insert a comma before the 'and' or 'or'." It lists the example: “Red, white, and blue.”

The University of Oxford style guide takes a more nuanced view, saying there is “no comma between the penultimate item in a list and ‘and’/‘or’, unless required to prevent ambiguity”.

“However, always insert a comma in this position if it would help prevent confusion,” it adds, using the example: “She left her money to her parents, Mother Theresa and the pope”.

The University of Cambridge agrees. Its style guide states: “An Oxford comma is one used before an 'and' at the end of a list and is used to avoid ambiguity.”