A tractor unit consists of a cab and a short chassis behind the cab that is used to hook up to a semi-trailer. All tractor units are fitted with a universal fifth wheel coupling to which all types of semi-trailers can be attached. Tractor units mainly differ in the cab design. A normal high cab or a low cab are two of the typical designs. A sleeper cab and a semi-sleeper cab obviously make it possible for the driver to spend the night in the truck. The DAF XF, Scania R series and Mercedes-Benz Actros are common types.