The four most common plows in the history of Southern agriculture are the walking plow, the sulky plow, the gang plow, and the tractor plow. The walking plow, usually pulled by mules, was guided by a plow man who walked behind the implement and held the plow handles. The sulky plow, invented by John Deere in 1875, had a seat and wheels that allowed the plowman to sit while mules pulled the tool across the field. The gang plow featured multiple bottoms, three wheels, and a seat that allowed a plowman to ride, and it was also drawn by horses or mules. The tractor plow, pulled by a tractor, revolutionized agriculture by allowing farmers to cut far more furrows than had been possible with earlier animal-drawn implements and by giving them far greater flexibility in the plow blades that could be attached. The cutting tools on a plow are commonly called bottoms, and agricultural historians commonly classify them into four groups: moldboard, which breaks up the soil, turns it to one side, and molds it; disk, which is designed to break up hard packed soil; chisel, a C-shaped plow that lifts soil without turning it over; and rotary plows, whose rotating blades mix manure and other nutrients into the soil.