An allocatively efficient economy produces an "optimal mix" of commodities.[1]:9 A firm is allocatively efficient when its price is equal to its marginal costs (that is, P = MC) in a perfect market. The demand curve coincides with the marginal utility curve, which measures the (private) benefit of the additional unit, while the supply curve coincides with the marginal cost curve, which measures the (private) cost of the additional unit. In a perfect market, there are no externalities, implying that the demand curve is also equal to the social benefit of the additional unit, while the supply curve measures the social cost of the additional unit.