In various countries, gold was used as a standard for monetary exchange, but this practice has been abandoned with the rise of fiat currency. The last country to back their money with gold was Switzerland, which backed 40% of its value until it joined the International Monetary Fund in 1999. Pure gold is too soft for ordinary use and is typically hardened by alloying with copper or other base metals. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k. Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness. Modern collector/investment bullion coins (which do not require good mechanical wear properties) are typically 24k, although the American Gold Eagle, the British gold sovereign and the South African Krugerrand continue to be made at 22k, on historical tradition. The
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