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).
その慣習は戦国時代(日本)初期の永正2年（1505年）の室町幕府による撰銭令の中において、（銅銭96枚を100文とすることを前提として）100文の1/3を32文として換算する規定が見られ、こうした慣行を九六銭（くろくせん）とも呼んだ。