"My interest in overhead costs was the structure of prices in situations where average cost per unit exceeds marginal cost. The Pareto rule was that price should equal marginal cost, but to apply this rule would bankrupt the firm. In practice, such situations oscillate between bankruptcy and monopoly, as in the airline industry today. The general inclination of economists in those cases was to enforce marginal pricing and subsidize firms to the extent of the differences between marginal and average cost. This was hardly practical, as an industry-wide policy. Neither could it be justified, as many taxpayers would be forced to pay for services that they did not use. If one started from the premise that those who use the service should pay for it, the problem reduced to how to spread the fixed costs among the users. Here I started from the railway principle of “charging what the traffic will bear” and linked up with the new price discrimination theory, as elaborated by Joan Robinson. Another aspect of overhead costs was the time dimension. Demand was not steady, but fluctuated. If the output could not be stored, there would be times of idle capacity, regular or irregular; how was the cost of this to be shared? I demonstrated that the correct approach to this problem was to treat the fixed investment as a producer in joint cost of different outputs at different times, each paying what it could bear, and subject to the sum of payments not exceeding total cost."