 The final series of paper money issued by the government of the Yuan dynasty from 1350 were the Zhizheng Jiaochao (至正交鈔).A major difference between how paper money was used under the Mongols and under the Song dynasty was that, in certain regions of the Yuan dynasty, paper notes were the only acceptable form of currency and could not be exchanged in either copper cash coins or silver sycees.Exchanging paper money into copper or silver was known as "duìxiàn" (兌現, "convert into specie") which was the main reason why earlier forms of paper money were deemed reliable.