where did the term 5th wheel come from
Fifth-wheel coupling The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. The device allowed the front axle assembly to pivot in the horizontal plane, to facilitate turning. Basically a wheel was placed on the rear frame section of the truck, which back then only had four wheels; this wheel that was placed on the frame was the "fifth wheel", hence the name. The trailer needed to be raised so that the trailer's pin would be able to drop into the central hole of the fifth wheel.
BMW 5 Series Initially, the 5 Series was only available in a sedan body style. The wagon/estate body style (called "Touring") was added in 1991 and the 5-door fastback (called "Gran Turismo") was added in 2009.