A currency in the most specific use of the word refers to money in any form when in actual use or circulation as a medium of exchange, especially circulating banknotes and coins. A more general definition is that a currency is a system of money in common use, under this definition, US dollars, British pounds, Australian dollars, and European euros are examples of currency. These various currencies are recognized stores of value and are traded between nations in exchange markets, which determine the relative values of the different currencies. Currencies in this sense are defined by governments, and each type has limited boundaries of acceptance, other definitions of the term currency are discussed in their respective synonymous articles banknote, coin, and money. The latter definition, pertaining to the systems of nations, is the topic of this article. Currencies can be classified into two systems, fiat money and commodity money, depending on what guarantees the value. Some currencies are legal tender in certain jurisdictions, which means they cannot be refused as payment for debt. Others are simply traded for their economic value, digital currency has arisen with the popularity of computers and the Internet. Currency evolved from two basic innovations, both of which had occurred by 2000 BC, originally money was a form of receipt, representing grain stored in temple granaries in Sumer in ancient Mesopotamia, then Ancient Egypt. In this first stage of currency, metals were used as symbols to represent value stored in the form of commodities and this formed the basis of trade in the Fertile Crescent for over 1500 years. Trade could only reach as far as the credibility of that military and it is not known what was used as a currency for these exchanges, but it is thought that ox-hide shaped ingots of copper, produced in Cyprus, may have functioned as a currency. It is thought that the increase in piracy and raiding associated with the Bronze Age collapse, possibly produced by the Peoples of the Sea, brought the trading system of oxhide ingots to an end. In Africa, many forms of value store have been used, including beads, ingots, ivory, various forms of weapons, livestock, the manilla currency, the manilla rings of West Africa were one of the currencies used from the 15th century onwards to sell slaves. African currency is still notable for its variety, and in many various forms of barter still apply. These factors led to the metal itself being the store of value, first silver, now we have copper coins and other non-precious metals as coins. Metals were mined, weighed, and stamped into coins and this was to assure the individual taking the coin that he was getting a certain known weight of precious metal. Coins could be counterfeited, but they created a new unit of account. Most major economies using coinage had several tiers of coins, using a mix of copper, silver, gold coins were used for large purchases, payment of the military and backing of state activities, they were more often used as measures of account than physical coins
Cowry shells being used as money by an Arab trader.
Song dynasty Jiaozi, the world's earliest paper money.