 In Europe, this system worked through the medieval period because there was virtually no new gold, silver or copper introduced through mining or conquest.Thus the overall ratios of the three coinages remained roughly equivalent.In premodern China, the need for credit and for circulating a medium that was less of a burden than exchanging thousands of copper coins led to the introduction of paper money, commonly known today as "banknote"s. This economic phenomenon was a slow and gradual process that took place from the late Tang dynasty (618–907) into the Song dynasty (960–1279).