Commas are used to separate items in lists, as in They own a cat, a dog, two rabbits and six mice. In English, a comma may or may not be used before the final conjunction (and, or, nor) in a list of more than two elements. A comma used in such a position is called aserial comma or an Oxford or Harvard comma (after the Oxford University Press and Harvard University Press, both prominent advocates of this style). In some cases, use or omission of such a comma may serve to avoid ambiguity:
Use of serial comma disambiguating:
- I spoke to the boys, Sam and Tom. – could be either the boys and Sam and Tom or the boys, who are Sam and Tom (I spoke to two people).
- I spoke to the boys, Sam, and Tom. – must be the boys and Sam and Tom.
Omission of serial comma disambiguating:
- I thank my mother, Anne Smith and Thomas.– The writer is thanking three people: the writer’s mother and Anne Smith (who is not the writer’s mother) and Thomas.
- I thank my mother, Anne Smith, and Thomas.– Could be eithermy mother and Anne Smith and ThomasorThomas and my mother, who is Anne Smith.
Hope this is helpful for you!