. . "A road tractor coupled to a semi-trailer is often called a semi-trailer truck or semi. Though most road trailers meet this definition, the term is most often applied to heavy trailers appropriate for use in such a rig. In Australian English, the tractor unit is usually referred to as a prime-mover and the combination of a prime-mover and trailer is known as a semi-trailer or semi. Semi-trailers with two trailer units are B-Doubles or Road Trains. A B-double consists of a prime mover towing two semi-trailers where the first semi-trailer is connected to the prime mover by a fifth wheel coupling and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling. A Road Train means a combination, other than a B-Double, consisting of a motor vehicle towing at least two trailers (counting as a single trailer a converter dolly supporting a semi-trailer)."@en . . . "Semi-trailers"@en . . . . . . "A road tractor coupled to a semi-trailer is often called a semi-trailer truck or semi. Though most road trailers meet this definition, the term is most often applied to heavy trailers appropriate for use in such a rig."@en .