The harrow is a mechanical device that is closely related to the plow. Whereas the plow does the initial breaking of the sod, the harrow further loosens the soil and also cuts field stubble. A triangular harrow, or "A" frame harrow, was first used in the 1600s. This harrow design remained almost unchanged until after the American Civil War. In 1869 David L. Garver of Michigan patented a spring tooth harrow. Widely used in regions such as the eastern United States where the soil is quite rocky, the spring tooth harrow uses spring steel teeth that flex when they strike an obstacle. The disk harrow was developed in the 1870s and was especially well received in the middle west and great plains states. Its use increased greatly in the 1890s and remained popular through World War II. Since the 1950s, use of disk harrows has decreased dramatically, with wider acceptance being given to cultivators that do the same job more efficiently. Disk harrows differ from disk plows in that the plows move the soil in one direction while the harrow moves it in opposing directions. Disk plows have three to six individually mounted disks. Disk harrows have rows of blades, called gangs, mounted on axles. The single-acting disk harrow has a pair of opposing blade gangs pulled by a tractor. The tandem disk harrow has two sets of opposing gangs. The offset disk harrow has one gang trailing another, in which the line of pull is far to one side of the till strip, making it favorable for tilling under low tree branches, as in orchards. Another type of harrow is the spike-toothed harrow. Used to smooth the soil after plowing, it performs the same function as a rigid-tined garden rake.