The Allis-Chalmers D series is a line of tractors made by the Allis-Chalmers Manufacturing Company from 1957 to 1969. Most of the D-series tractors, except for the D21, had hand-operated, shift-on-the-go oil clutches, commonly referred to as a hand clutch. Allis-Chalmers marketed this arrangement as the "Power Director". This clutch was in low range when pulled back, neutral in the middle, and high range when pushed forward. It could also be ordered as a forward-reverser, called the "Shuttle Clutch". This hand clutch also exists on the D21, but it was not meant for shifting on the go.