Tanhaku (also called shohaku) is a business custom which had been conducted in East Asia before the early modern times; with this custom, a bundle of copper coins consisting of a certain number of coins less than 100 is treated as equal value to 100 copper coins.
Since the period of the Tang Dynasty, copper coins issued by the Chinese dynasty were prevailed with high creditworthiness, so Japan and other neighboring countries started using them instead of the local currency.
However, since Chinese copper production capacity was not high enough and a demand for copper coins increased at a pace surpassing the amount of copper coins issued due to rapid economic development, a phenomenon called senko that the quantity of circulating copper coins was chronically short occurred.
Thus, the actual value of the copper coin became higher than the official value and it had a major impact on the economy. In this way, the custom of tanhaku, in which a bundle of a certain number of copper coins held together by a piece of string was considered to be 100 coins, became established from around the end of the Tang Dynasty.
This meant actual revaluation of currency and at the same time, it became established as a favorable system for merchant princes who handled a great number of copper coins. Osho (王章) (Five Dynasties period) who was the prime minister of the Later Han Dynasty (Five Dynasties period), one of Godai-Jikkoku (Wudai Shiguo) period, officially permitted to consider 77 copper coins equal to 100 and it was also adopted in the dynasties after the Sung. For civilians, the rate of tanhaku with fewer coins was established. A payment made with 100 copper coins instead of tanhaku is called '足銭' in Chinese and 'chosen' (長銭) or 'chohaku' (調陌) in Japanese.
Tanhaku was a custom fit for the economic situation at that time and hailed as a method to reduce the inconvenience of delivering copper coins. However, while it led to the increase in property value of merchant princes who owned a lot of copper coins and carried out trading, it was a very unfavorable system for ordinary people whose trading was mostly with a small amount of money in daily life because if tanhaku wasn't adopted, the value returned to the primary official value of the copper coin (if it was calculated based on the official value in the age of Sung, the total value of coppers without tanhaku would be just the value of 77 coins, thus it resulted in a loss of 23 coins).
In Japan, it is said that there was a business custom with which 97 copper coins were considered as 100 coins in the Muromachi period. The custom with which a bundle of 96 copper coins (equal to 96 mon [an old currency unit in Japan]) was considered as 100 coins in the Edo period is regarded as one of tanhaku. This custom is found in the ban on use of poor-quality currency issued by the Muromachi bakufu (Japanese feudal government headed by a shogun) in the early Sengoku period (period of warring states) (Japan), 1505, saying (on the assumption that 96 cooper coins is considered as 100 mon) all figures must be converted at 32 mon to one third of 100 mon, and such a custom was also called kurokusen ("ku" means 9, "roku" means 6 and "sen" means money).