If the exchange value of one yard of linen is expressed in 1/2 lb. of tea, or 2 lbs. of coffee, or 6 yards of calico, or 8 lbs. of bread, etc., it follows that coffee, tea, calico, bread, etc., are equal to each other if taken in the same proportion in which they are equal to the third article, linen; consequently, linen serves as the common measure of their exchange values. Every commodity, as the embodiment of universal labor-time, i. e., as a certain quantity of universal labor-time, expresses in turn its exchange value in definite quantities of the use-values of all other commodities, and the exchange values of all the other commodities are, on the other hand, measured by the use-value of this one exclusive commodity. But as an exchange value, every commodity is at the same time the one exclusive commodity that serves as a common measure of the exchange values of all other commodities; and, on the other hand, it is but one of the many commodities in the entire series of which every commodity expresses directly its exchange value. The value of a commodity is not affected by the number of commodities of other kinds. But the length of the series of equations in which its exchange value is realized does depend upon the greater or less variety of other commodities. The series of equations in which the value of coffee, e. g., is represented, indicates the extent to which it is exchangeable, the limits within which it performs the function of an exchange value. The exchange value of a commodity as an embodiment of universal social labor-time is expressed in its equivalence to an endless variety of use-values. We have seen that the exchange value of a commodity varies with the quantity of labor-time directly contained in it. Its realized exchange value, i. e., its exchange value expressed in the use-values of other commodities, must also depend on the proportion in which the labor-time spent on the production of all other commodities is changing. If, e. g., the labor-time required for the production of a bushel of wheat remained constant, while that required for the production of all other commodities doubled, the exchange value of a bushel of wheat expressed in its equivalents would become half as large as before. The result would be practically the same as if the amount of time necessary for the production of one bushel of wheat had been reduced by one-half, and that required for all other commodities had remained unchanged. The value of commodities is determined by the proportion in which they can be produced in the same labor-time. In order to see what possible changes this proportion may undergo, let us take two commodities, A and B. _First case._ Let the labor-time required for the production of commodity B remain unchanged. In that case the exchange value of A, expressed in terms of B, rises and falls with the rise and fall of the labor-time required for the production of A.