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Should I put a comma before the last item in a list?

How do I know if I need to add a comma or not before a conjunction preceding the last item in an enumeration?

For example, which of the following sentences is correct?

I have a house, a car, a bicycle, and an airplane.

I have a house, a car, a bicycle and an airplane.

Are there any free online references on this matter (with examples, if possible) for the layman?



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