In the thirteenth century, Kublai Khan completed the Mongol conquest of China, initiated by his grandfather Genghis Khan. Kublai established the Yuan Empire and made its capital modern-day Beijing. At that time in history, metal coins were mainly used for trading and purchasing, though paper money had been around since the Song dynasty. Kublai made paper money (Chao) the standard for exchange. The supply of copper was inadequate for the production of metal currency as global trade continued to expand. The Book of Ser Marco polo, translated and edited by Sir Henry Yule, contains an excerpt in which Marco Polo describes the use of paper money by the Mongols to his fellow Europeans. Marco Polo and other Europeans were fascinated by how the Mongols made paper out of trees and how the king essentially acquired all the wealth in the empire. The creation and advancement of paper currency helped make exchange easier and led to its adoption by other regions later on in history. Though this method became essential in Mongol society, there were plenty of problems with paper currency. Paper currency was easier to make and the production costs were much lower than the cost of producing metal coins. It could also be printed at any time for any purpose which became helpful at times of domestic crises and warfare. Foreign merchants possessing gold or silver were prohibited form exchanging with anyone by Kublai himself. The merchants accepted the paper notes because they got more value from Kublai than they would anywhere else. The notes could be used to purchase anything over the empire as if they were coins made of pure gold, which made it an incentive to exchange coins for paper money. Paper