The skeleton of a hand-knotted rug: the warp and weft threads on which every pile knot is tied. Cotton is the usual material in city and village rugs because it barely stretches or shrinks with humidity, so the rug keeps its shape, and it is rigid enough to knot through easily; that same rigidity can carry over into the design, which reads as less fluid than on a wool foundation. Wool, and sometimes goat hair, is typical of tribal rugs, whose weavers can use one material for both foundation and pile; it gives more character but a less straight shape. Silk, being finer, permits very high knot counts. The fringes at each end are the foundation's exposed warp ends, so they reveal its material.