Because soils rarely consist of particles of only one size, textural categories have been established based on the varying proportions of clay, silt, and sand. The standard system of classes used by the U.S. Department of Agriculture is shown in Figure 413. For example, point A on this triangular diagram(left center) represents a soil composed of 10 percent silt, 40 percent clay, and 50 percent sand. Such a soil is called a sandy clay.