by Jeff Thomas, International Man:

In 1260, Kublai Khan created the first unified fiat currency. The jiaochao was made from the inner layer of bark of the mulberry tree. It’s of interest that the mulberry tree was quite common in Mongolia. What allowed Kublai Khan to get away with treating tree bark as currency was that each bill was cut to size and signed by a variety of officials. They affixed their seals to each bill. To further ensure authenticity, forgery of the chao was made punishable by death.