A drivetrain system in which torque generated by the engine is delivered to only one pair of wheels at a time, either the two front or two rear wheels. Thus there are exactly two driven wheels, while the other two wheels merely roll under the momentum imparted to them and do not create tractive force of their own. This is in contrast to four-wheel drive (4WD), in which all four wheels receive power simultaneously. Some vehicles are only capable of two-wheel drive such that it is permanent, while in others it is a selectable mode, with the vehicle capable of switching between two-wheel drive and four-wheel drive on-demand.