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Why do they call 18 wheelers semis?

18 wheelers are not called semis. They are called semis because of the semi-trailer they pull behind them. A semi-trailer is a freight trailer that is not self-propelled and is designed to be pulled by a truck. The term "semi" comes from the Latin word "semi," meaning "half." It refers to the fact that a semi-trailer is only half of a complete trailer. The other half is the truck that pulls it.