Pull-type chisel ploughs are made in working widths from about 2.5 m (8 ft) up to 13.7 m (45 ft). They are tractor mounted, and working depth is hydraulically controlled. Those more than about 4 m (13 ft) wide may be equipped with folding wings to reduce transport width. Wider machines may have the wings supported by individual wheels and hinge joints to allow flexing of the machine over uneven ground. The wider models usually have a wheel each side to control working depth. Three-point hitch-mounted units are made in widths from about 1.5 m to 9 m (5–30 ft).