What is the ‘Oxford comma’? – Oxford Dictionaries (US).
The ‘Oxford comma’ is an optional comma before the word ‘and’ at the end of a list:
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It’s known as the Oxford comma because it was traditionally used by printers, readers, and editors at Oxford University Press. Not all writers and publishers use it, but it can clarify the meaning of a sentence when the items in a list are not single words:
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The Oxford comma is also known as the ‘serial comma’.