In economics, Gresham's law is a monetary principle stating that "bad money drives out good". For example, if there are two forms of commodity money in circulation, which are accepted by law as having similar face value, the more valuable commodity will gradually disappear from circulation.
Sir Thomas Gresham
A stack of twenty Walking Liberty half dollars (left), which contain 90% silver. In an example of Gresham's law, these coins were quickly hoarded by the public after the Coinage Act of 1965 debased half dollars to contain only 40% silver, and then were debased entirely in 1971 to base cupronickel (right).
Commodity money is money whose value comes from a commodity of which it is made. Commodity money consists of objects having value or use in themselves as well as their value in buying goods.
This is in contrast to representative money, which has no intrinsic value but represents something of value such as gold or silver, in which it can be exchanged, and fiat money, which derives its value from having been established as money by government regulation.
Japanese commodity money before the 8th century AD: arrowheads, rice grains and gold powder. This is the earliest form of Japanese currency.
A bronze okpoho or manilla, the traditional commodity money in West Africa until the 1940s.
Axe-like grzywnas (commodity money) from Kostkowice, Poland, 9th to mid-10th century AD