However, with the elimination of Kocho-Junisen (twelve coins casted in Japan), circulation of currency fell, and on the other hand, with the wide-spread use of in-kind tax payments and purchasing goods from the trade related to the in-kind tax payments to deliver to the central government, koka was called for as the standards for prices.
だが、皇朝十二銭の廃絶などで貨幣流通が衰退する一方で代納品による租税の物納やそれに伴う交易による中央への上供品の購入が一般化すると、交換の価値基準としての估価が求められるようになった。