made their first appearance later, in the Sung period. A fourth important innovation, this time in North China, was the introduction of prototypes of paper money. The Chinese copper "cash" was difficult or expensive to transport, simply because of its weight. It thus presented great obstacles to trade. Occasionally a region with an adverse balance of trade would lose all its copper money, with the result of a local deflation. From time to time, iron money was introduced in such deficit areas; it had for the first time been used in Szechwan in the first century B.C., and was there extensively used in the tenth century when after the conquest of the local state all copper was taken to the east by the conquerors. So long as there was an orderly administration, the government could send it money, though at considerable cost; but if the administration was not functioning well, the deflation continued. For this reason some provinces prohibited the export of copper money from their territory at the end of the eighth century. As the provinces were in the hands of military governors, the central government could do next to nothing to prevent this. On the other hand, the prohibition automatically made an end of all external trade. The merchants accordingly began to prepare deposit certificates, and in this way to set up a sort of transfer system. Soon these deposit certificates entered into circulation as a sort of medium of payment at first again in Szechwan, and gradually this led to a banking system and the linking of wholesale trade with it. This made possible a much greater volume of trade. Towards the end of the T'ang period the government began to issue deposit certificates of its own: the merchant deposited his copper money with a government agency, receiving in exchange a certificate which he could put into circulation like money. Meanwhile the government could put out the deposited money at interest, or throw it into general circulation. The government's deposit certificates were now printed. They were the predecessors of the paper money used from the time of the Sung. 4 _Political history of the Five Dynasties_