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Types of trucks by size
Light trucks are trucks the size of cars. They are used by individuals and also companies. In the United States, a truck is a light truck if it weighs less than 6,300 kg (13,000 lb). Light trucks are only a little heavier than vans or pickup trucks, but require a special driver's license.
Medium trucks are heavier than light trucks but lighter than heavy trucks. In the United States, a truck is a medium truck if it weighs between 6,300 kg (13,000 lb) and 15,000 kg (33,000 lb). Trucks that are used for local delivery and public service (dump trucks, garbage trucks) are normally around this size. Medium trucks have usually two axles: one at front and one at rear. The rear wheels may be coupled (that is: two wheels hitched together) to enable heavier load, but they may also be single.
Heavy trucks are the heaviest trucks that are allowed on the road. In the U.K. they are known as lorries. Usually heavy trucks have three axles: one at front and two at rear, and the rear axles have coupled wheels to enable heavy loads. Often heavy trucks pull trailers. They can be either full trailers which have both front and rear axles or boggies and their own brakes, and are connected to the towing truck with a towbar, or semi-trailers, which are attached to a special kind of a truck called tractor unit with a turntable coupling ("fifth wheel"). A semi-trailer is a kind of trailer which has wheels only in the back and the front rides on the back of the tractor unit. The tractor unit has an engine and the semi-trailer does not. Driving a heavy truck requires a professional driver's license. Truck drivers are called truckers.
The laws of various countries say what kind of vehicle combinations are allowed. A semi-trailer can be converted into a full trailer with using a dolly. A dolly is a small trailer which has only a bogie (= set of wheels), fifth wheel coupling for the semi-trailer and a towbar. Using a dolly, the semi-trailer can now be hitched to an ordinary cargo-carrying truck and does not need a tractor unit.
A heavy truck-trailer combination is often called big rig or eighteen-wheeler.