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So if I go out to eat with a party of 15 and they automatically add 15% to the bill and I don't tip on top of that, I've stiffed them?

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Probably. 15% is a marginal tip, particularly for someone serving a big party. And as long as we are into silly semantics, the 15% is usually referred to on the menu as a service charge. And since it is not optional, it is not a tip.