Standards of value
In the Middle Ages, when money consisted primarily of coins, silver and gold coins circulated simultaneously. As governments came increasingly to take over the coinage and especially as fiduciary money was introduced, they specified their nominal (face value) monetary units in terms of fixed weights of either silver or gold. Some adopted a national bimetallic standard, with fixed weights for both gold and silver based on their relative values on a given date—for example, 15 ounces of silver equal 1 ounce of gold (see bimetallism). As the prices changed, the phenomenon associated with Gresham’s law assured that the bimetallic standard degenerated into a monometallic standard. If, for example, the quantity of silver designated as the monetary equivalent of 1 ounce of gold (15 to 1) was less than the quantity that could be purchased in the market for 1 ounce of gold (say 16 to 1), no one would bring gold to be coined. Holders of gold could instead profit by buying silver in the market, receiving 16 ounces for each ounce of gold; they would then take 15 ounces of silver to the mint to be coined and accept payment in gold.
Continuing this profitable exchange drained gold from the mint, leaving the mint with silver coinage. In this example silver, the cheaper metal in the market, “drove out” gold and became the standard. This happened in most of the countries of Europe, so that by the early 19th century all were effectively on a silver standard. In Britain, on the other hand, the ratio established in the 18th century on the advice of Sir Isaac Newton, then serving as master of the mint, overvalued gold and therefore led to an effective gold standard. In the United States a ratio of 15 ounces of silver to 1 ounce of gold was set in 1792. This ratio overvalued silver, so silver became the standard. Then in 1834 the ratio was altered to 16 to 1, which overvalued gold, so gold again became the standard.