Long-run is a period of time in which all the factors of production are variable. Thus, in the long- run, the output can be increased by increasing capital equipment or by increasing the size of the existing plants or by building a new plant of greater production capacity. The long-run cost is the cost at which the firm operates in the long-run. The long-run cost curve is also called the planning curve because it shows optimal possibilities for expansion of output and thus helps the entrepreneur to plan his future investment activities. Before an investment is decided, the entrepreneur can choose any of a wide range of alternative investment defined by the state of technology.