The wheel or wheels used to transform torque from the powertrain into tractive force, and to transmit that force from the tires to the road, overcoming stationary forces and causing the vehicle to move forwards or backwards; i.e. the wheels that are driven, as opposed to merely rolling under the momentum imparted to them. Vehicles may have any combination of driven wheels; power applied to the two front wheels or two back wheels is two-wheel drive (2WD), and power applied to all four wheels is four-wheel drive (4WD). Which wheels are driven is variable in all-wheel drive (AWD).