<P> Many cultures around the world eventually developed the use of commodity money . The Mesopotamian shekel was a unit of weight, and relied on the mass of something like 160 grains of barley . The first usage of the term came from Mesopotamia circa 3000 BC . Societies in the Americas, Asia, Africa and Australia used shell money--often, the shells of the cowry (Cypraea moneta L. or C. annulus L .). According to Herodotus, the Lydians were the first people to introduce the use of gold and silver coins . It is thought by modern scholars that these first stamped coins were minted around 650--600 BC . </P> <P> The system of commodity money eventually evolved into a system of representative money . This occurred because gold and silver merchants or banks would issue receipts to their depositors--redeemable for the commodity money deposited . Eventually, these receipts became generally accepted as a means of payment and were used as money . Paper money or banknotes were first used in China during the Song dynasty . These banknotes, known as "jiaozi", evolved from promissory notes that had been used since the 7th century . However, they did not displace commodity money, and were used alongside coins . In the 13th century, paper money became known in Europe through the accounts of travelers, such as Marco Polo and William of Rubruck . Marco Polo's account of paper money during the Yuan dynasty is the subject of a chapter of his book, The Travels of Marco Polo, titled "How the Great Kaan Causeth the Bark of Trees, Made Into Something Like Paper, to Pass for Money All Over his Country ." Banknotes were first issued in Europe by Stockholms Banco in 1661, and were again also used alongside coins . The gold standard, a monetary system where the medium of exchange are paper notes that are convertible into pre-set, fixed quantities of gold, replaced the use of gold coins as currency in the 17th--19th centuries in Europe . These gold standard notes were made legal tender, and redemption into gold coins was discouraged . By the beginning of the 20th century almost all countries had adopted the gold standard, backing their legal tender notes with fixed amounts of gold . </P> <P> After World War II and the Bretton Woods Conference, most countries adopted fiat currencies that were fixed to the U.S. dollar . The U.S. dollar was in turn fixed to gold . In 1971 the U.S. government suspended the convertibility of the U.S. dollar to gold . After this many countries de-pegged their currencies from the U.S. dollar, and most of the world's currencies became unbacked by anything except the governments' fiat of legal tender and the ability to convert the money into goods via payment . According to proponents of modern money theory, fiat money is also backed by taxes . 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Which money is most economical in terms of its cost