Gresham's Law of Coinage

Gresham's law is an economic principle that states: "When a government compulsorily overvalues one type of money and undervalues another, the undervalued money will leave the country or disappear from circulation into hoards, while the overvalued money will flood into circulation." It is commonly stated as: "Bad money drives out good", but is more accurately stated: "Bad money drives out good if their exchange rate is set by law."

It can also be applied to the consequences of government price controls for example a possible outcome of setting a maximum price for tomatoes might be that only poor quality tomatoes are offered for sale at that maximum price while good quality tomatoes are sold on the black market at whatever price the market will bear.