A serial comma is a comma placed before a conjunction in a series. Style guides disagree on its usage; however American newspapers rarely use it, as it wastes space. The AP stylebook dictates that commas should be placed before conjunctions in a series only if a series item contains a conjunction (like peanut butter and jelly - which is one item) or if the series is complex. Sometimes, a serial comma can remove ambiguity, such as in the sentence "This book is dedicated to my roommates, Nicole Kidman, and God." Were the serial comma to be removed, the sentence could be read to imply that Kidman and God were both the writer's roommates. Other times, the comma can create ambiguity, such as in the series "Betty, a maid, and a rabbit". This sentence could be read to mean that Betty is a maid, or the maid could be the second item in the series.
- Richard Nordquist. What is the Oxford (or Serial) Comma? (English). About.com.