An Oxford, or serial, comma is the last comma in a list, which goes before the word “and.” It’s, technically, grammatically optional in American English.
However, depending on the list you are writing out, omitting it can lead to some confusion. A Reddit user found what could be the best three sentence argument in existence for the use of the Oxford comma:
It’s likely the book is dedicated to the author’s parents, Ayn Rand, and also God — but then again, it could be that the author’s parents are named Ayn Rand and God.
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