Confidence in the value of fiat money is easily and quickly undermined under conditions of political or economic instability, especially when the government has already been observed reneging on some of its many other solemn promises and obligations and when the money stock is clearly being expanded at an extremely rapid rate -- with the result that a number of such national currencies have become worthless virtually overnight in the course of the 20th century's all too numerous war- and revolution-related hyperinflations.
[See also: barter , transaction costs , money stock , monetary policy , inflation , hyperinflation , deflation , banking ]