Legal tender laws would merely complicate trading, but practically always specify a conversion ratio - a price - between the privileged money and other monies, different from the market price. This is done to benefit debtors (like the state).
For example, the law sets the ratio at 20 ounces of silver for one ounce of gold, but the market price is 15. A contract or debt for 20 ounces of silver can be now paid with 1 ounce of gold - which the debtor can buy for much less than was the original debt. As a result, no one will be willing to enter a contract quoted in silver.
This is called Gresham's Law: an overvalued money will drive undervalued money out of the market. Or put simply, "the bad money drives the good money out". People stop using the metal, that is in reality more valuable than according to the letter of the law. The metal will be held or sold into other countries.
Bimetallism is a currency system with forced exchange ratios between coins from different metals, often between gold and silver. In accord with the Gresham's Law, if a metal is undervalued, it will be driven out of circulation. This was notable, for example, in the British currency reform of 1717, or the US reforms of 1792 and 1834.
In 1792, the U.S. Congress voted a Coinage Act into existence that decreed the exchange rate between gold and silver to be 1 to 15. The market rate was 1 to 15.5, however, and after a few years the artificially undervalued gold had all but disappeared from circulation.
The U.S. Coin Act of 1834 fixed the legal exchange ratio between gold and silver at 1 to 16, and the entire silver currency of the country was replaced with a gold currency. Fractional reserve banking has benefited in both exchanges.
- Jörg Guido Hülsmann. "The Ethics of Money Production" (pdf), Chapter 10. Legal-Tender Laws p.125-131, referenced 2009-06-24.
- Jörg Guido Hülsmann. "Legal Tender Laws and Fractiona-Reserve Banking" (pdf), referenced 2009-11-08.
- Ludwig von Mises. "19. The Gold Standard ", online version of Human Action, referenced 2010-05-23.