In the early ninth century the Tang dynasty ran short of copper coin: a tax reform that accepted part of the taxes in money raised demand for cash, and coin was scarce from 805 to 820. Carrying strings of coin between the regions and the capital was a burden for merchants.
By 804, merchants were using a better way. They entrusted their money to the representative offices of their local governments (and to armies, commissioners and wealthy families) and carried a certificate instead. When the tallies were matched at an office, they could withdraw their money; at the capital the exchange fee was 100 wén per 1,000. Tea merchants, trading between the capital and the regions, benefited most.
People called it 飛錢, feiqian: "flying money". The value travelled; the coins stayed put. The government was wary at first, but in 812 flying cash was officially accepted as a means of exchange.