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Experts don't agree about this, but when in doubt, it is safer to use the serial comma. ("red, white, and blue" instead of "red, white and blue"). One of the reasons that it's safer to use this comma is to ensure clarity, especially in technical or legal writing. There was a court case where there was disagreement as to how the money should be divided among 3 people because of the lack of the serial comma. (The will said that "Bob, Betty and Jane" were to split the money. Because of the missing serial comma, there was confusion as to whether Bob got 50% and Betty & Jane split the other 50%, or whether each person got 1/3.) So to be safe, always use it.