Gresham's law

An economic principle proposed by an English financier, Sir Thomas Gresham, that bad money will drive good money out of circulation. For example, if the U.S. government minted silver dollars and then, at a later date, began to mint dollar coins out of cheaper metals, the public would hoard the silver dollars (possibly for later sale at higher prices) rather than use them as a medium of exchange: silver dollars would stop circulating.