In standard microeconomic theory, short-run and long-run marginal costs are equal for production equipment with adjusted capacity. When the production of joint products from interdependent equipment is modeled with a linear program, this equality is no longer verified. The short-run marginal cost then takes on a left-hand value and a right-hand value which generally differ from the long-run marginal cost. In this article, we demonstrate and interpret the relationship existing between long-run marginal cost and shortrun marginal costs for a given finished product. That relationship is simply expressed as a function of marginal capacity adjustments (determined in the long run) and marginal values of capacities (determined in the short run).