There are six key attributes to a useful currency: scarcity, divisibility, acceptability, portability, durability, and resistance to counterfeiting (uniformity). These qualities allow a currency to find widespread use in an economy. They also limit monetary inflation and ensure that the currencies are secure and safe to use.
Currency is useful if it works as a [store of value or, to put it differently, if it can reliably maintain its relative value over time. Throughout history, many societies used commodities or precious metals as methods of payment because they were considered to have a relatively stable value.
Rather than carry around cumbersome quantities of cocoa beans, gold, or other early forms of money, societies eventually turned to minted currency as an alternative. The first such currencies used metals like gold, silver, and bronze, which had long shelf lives and little risk of depreciation.1
Assigning value to currencies is a matter of debate. Initially, their value came from intrinsic physical properties. For example, gold’s value comes from the costs of extraction and certain qualitative factors, such as luster and purity content.
In the modern age, government-issued currencies often take the form of paper money, which does not have the same intrinsic scarcity as precious metals. For a long time, the value of paper money was determined by the amount of gold backing it. Even today, some currencies are “representative,” meaning that each coin or note can be directly exchanged for a specified amount of a commodity.
The idea of a currency’s value began changing in the 17th century. Prominent Scottish economist John Law wrote that money—currency issued by a government or monarch—"is not the value for which goods are exchanged, but the value by which they are exchanged."2 In other words, the value of a currency is a measure of its demand and its ability to stimulate trade and business within and outside an economy.
This thinking hews closely to the modern credit theory for monetary systems. In this theory, commercial banks create money (and value for currencies) by lending to borrowers, who use the money to purchase goods and cause currency to circulate in an economy.