グレシャムの法則
The money used by a community can change on a smaller scale. This can come through innovation, such as the adoption of cheques (checks). Gresham's law says that "good money drives out bad". That is, when buying a good, a person is more likely to pass on less-desirable items that qualify as "money" and hold on to more valuable ones. For example, coins with less silver in them (but which are still valid coins) are more likely to circulate in the community. This may effectively change the money used by a community.