That’s the comma that sometimes appears just before the coordinating conjunction (normally and or or) near the end of a list of three or more items. There’s one in the title of this post. It became known as the Oxford comma because “for a century it has been part of Oxford University Press style to retain or impose this last comma consistently”.
Its omission often makes little difference (“We offer tea, coffee and orange juice”), but ambiguity arises easily (“At Jim’s house I met Jo, a student and an artist”). An almighty fuss broke out among writers and editors on Twitter this week when it emerged that a style guide for University of Oxford staff advises against using the Oxford comma, except where it “would assist in the meaning of the sentence or helps to resolve ambiguity”.
Mark Allen pointed out that the page in question was last updated in 2009, but it seemed to have slipped under the collective editorial radar until lately. Some people thought that Oxford University Press, or even the Oxford Manual of Style, were abolishing their eponymous mark. Not so.
Yet there was much gnashing of teeth, wailing and flailing, and references to “cold, dead hands”. I saw an astonishing number of people mourning the “death” of the Oxford comma.