The Oxford Comma is an optional comma before the word and at the end of a list.
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It is so called because it was traditionally used by printers, readers, and editors at Oxford University Press. Sometimes it can be necessary for clarity when the items in the list are not single words:
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Some people do not realize that the Oxford Comma is acceptable, possibly because they were brought up with the supposed rule (which Fowler would call a superstition) about putting punctuation marks before and.