The shape of the long-run average cost curve is related to economies and diseconomies of scale. Economies of scale are factors that cause long-run average cost to fall as output expands. Diseconomies of scale are factors that cause long-run average cost to rise as output expands. The minimum efficient scale is the lowest rate of output at which long-run average cost is at a minimum. Constant long-run average cost may occur at some plant sizes above the minimum efficient scale. Eventually, diseconomies of scale become powerful enough to cause rising long-run average cost.