Cotton threads are stretched vertically from one end of a loom to the other, (these are called warps) are passed alternately over and under the warps to create a flat-weave rug. If strands of yarn are knotted around each warp thread while the rug is being woven, a pile rug is created. It is this pile that provides both the thickness and the pattern of an oriental rug. The fineness and quality of each rug is determined by factors such as the number of knots per square foot, (the knot density).As the knot density increases, more intricate designs and greater detail can be achieved. Also, the more knots per square foot there are, the shorter the pile yarns can be cut. Hence, the highest quality rugs are those with fine patterns and thin piles.