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Trucks are divided into light-duty, medium-duty, and heavy-duty classifications depending on their weight. Heavy-duty trucks have a gross vehicle weight of 33,000 lb (15,000 kg) or more (i.e. the weight of the vehicle plus the weight of the payload is 33,000 pounds or more). When a heavy-duty truck is pulling a trailer, it may have a gross combination weight of 80,000 lb (36,360 kg) or more.
Technically, a vehicle that carries the load by itself, without a trailer, is known as a truck, or a straight truck. Examples include certain dump trucks, concrete mixers, and garbage trucks. A vehicle that pulls the load in a trailer is known as a tractor. The tractor is coupled to the trailer through a pivot point, known as the fifth wheel, which is mounted on top of the tractor frame. Most of the big rigs on highways are tractors pulling trailers.