A semi-trailer truck (also called an articulated lorry, tractor-trailer, or simply semi) is a large freight vehicle consisting of two main parts: . Tractor unit – the front part with the engine, driver’s cab, and the coupling device (fifth wheel). Semi-trailer – the rear load-carrying unit without a front axle, supported at the front by the tractor unit and at the rear by its own wheels.
A Mercedes semi-trailer truck. (Regensburg, Germany) © Wikimedia.org/High Contrast, CC BY-DE