The long run total cost curve is the lower cost envelope of all possible short run total cost options. Figure 6.6.1 illustrates the situation in which two possible infrastructure capacity options exist (which might represent different size components, an additional floor on a parking garage or an additional lane of roadway capacity). There is a usage point ($$q_b$$) in Figure 6.6.1 at which it is desirable to shift from option 1 to the larger capacity option 2. The long run average total cost has a significant turn at this usage breakpoint, with scale economies beginning again! The long run marginal cost curve has a discontinuity at the breakpoint. With lower demand (demand curve $$D$$), the smaller option has lower costs, but with higher demand (demand curve $$D’$$), the larger option 2 is more desirable.