Gold as Medium of Monetary Exchange

Throughout the world gold was widely used as a standard for monetary exchange, but has been abandoned by world governments which have issued fiat currency in its stead. The amount of gold in the world is finite, but there is no limit to the quantity of paper currency which can be issued. At the beginning of World War I the warring nations moved to a fractional gold standard, inflating their currencies to finance the war effort. After World War II gold was replaced by a system of convertible currency following the Bretton Woods system. The last country to tie its currency to gold was Switzerland, which backed 40% of its value until the Swiss joined the International Monetary Fund in 1999.[8]
Pure gold is too soft for day to day monetary use and was typically hardened by alloying with copper, silver or other base metals prior to the advent of paper money. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k. Many holders of gold in storage (as bullion coin or bullion) hold it as a hedge against inflation or other economic disruptions. (The ISO currency code of gold bullion is XAU).
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 special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin, at 99.999% (.99999 fine). The popular issue Canadian Gold Maple Leaf coin has a purity of 99.99%. Several other 99.99% pure gold coins are currently available, including Australia's Gold Kangaroos (first appearing in 1986 as the Australian Gold Nugget, with the kangaroo theme appearing in 1989), the several coins of the Australian Lunar Calendar series, and the Austrian Philharmonic. In 2006, the U.S. Mint began production of the American Buffalo gold bullion coin also at 99.99% purity.

Source: Wikipedia