(From the entry on “Money,” by Charles Francis Bastable, in the famous 11th edition of the Encyclopedia Britannica):

The very large number of the autonomous cities of Greece, which possessed the right of issuing money, was the cause of the competition between different currencies, each having legal tender power only within its own city. In its practical outcome this “free coinage” system proved beneficial, for it compelled the maintenance of the true standard in order to gain wider circulation. With the establishment of larger states the control over the issue of money grew more stringent. In the later Roman Empire the right of coining was reserved to the emperor exclusively. …

A long course of debasement is the characteristic aspect of the [imperial Roman] currency system. “Under the empire,” we are told, “the history of silver coinage is one of melancholy debasement. The most extensive frauds in connexion with money were perpetuated by the Romans.” The gold aureas, which in the time of Augustus was one forty-fifth of a pound, was under Constantine only one seventy-second of a pound. The alloy in the silver coins gradually rose to three-fourths of the weight. Plated coins came into extensive use. The practice of debasement was in accordance with the theories of the jurists, who seem to have regarded money as simply the creature of the state.