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A tractor-trailer is the term given to a combination of a trailer and an on-road tractor, or truck. A tractor-trailer is comprised of two entirely separate units so that the truck can be hooked to other trailers and the trailer can be hooked to other trucks. They are connected both with hardware and electronically so that the truck can control the brakes and all the lights on the trailer.
The Tractor-Trailer is used for transporting goods and materials from one place to the other. It is how groceries find their way to the shelves of the grocery store, for one example.
The truck portion of a tractor-trailer must have enough engine power to pull a significant amount of weight. The weight of the trailer can vary greatly depending on what type of freight it contains. A trailer loaded with empty plastic bottles, for example, will weigh much less than a trailer full of bottled water.
Sometimes the passenger cab of the truck includes a compartment large enough for a bed. This is so the driver of the tractor-trailer can sleep in the truck when delivering freight over long distances. It is common for a driver to own his truck and contract out to companies to transport freight in the company trailers. One driver may haul freight for a number of different companies over a wide geographic area and spend weeks at a time living in his truck.
The trailer portion of the tractor-trailer combination can be one of a variety of designs to serve a variety of different purposes. Probably the most common style is the box trailer. It can vary in length from about 28 feet to 53 feet (8.5 to 16.2 meters) and is totally closed except for the door at rear end and sometimes on the sides.
Another style of trailer is known as a flat bed, which is basically a platform on which freight is strapped. A tractor-trailer with a flatbed is often used to transport lumber, bricks or other construction materials. A third common type of trailer is a tanker, which carries liquids such as water or milk or dry goods such as flour or grain.
Other terms used to describe a tractor-trailer are "eighteen wheeler" and semi-trailer truck." The term "eighteen wheeler" is sometimes used because there is often a total of eighteen wheels in the combination. The term "semi-trailer truck" refers to a trailer in which one end is supported by the truck that it is connected to. The other end of the trailer has its own set of wheels.