When the interaction of the physical quantities and the unit prices of a given commodity is marked by regularities over time, whether in the recurrence of cyclical or seasonal patterns or in the persistence of secular relations, it is convenient to define the differential movement of quantity with respect to price by a coefficient of elasticity, of price with respect to quantity by a coefficient of flexibility. These are mathematically identical with the familiar coefficients of Marshall and Moore, but in this usage they relate to designated temporal frameworks. The examples presented in the present paper deal with price-quantity interrelations in business cycles, in specific cycles in physical quantities, in specific cycles in unit prices, and in homogeneous secular periods. No attempt is made to eliminate or hold constant the dynamic influences that play upon the market. In each case we seek to measure the differential responsiveness of the quantity and price factors to the various impinging forces that operate within the temporal framework in question. If there are persistent regularities in the operation of these forces, appropriately designated coefficients of elasticity and flexibility can serve as useful instruments in the analysis of economic change.