kōchō jūnisen (皇朝十二銭) (aka jūnizeni) is a general term for 12 types of copper coins minted in Japan under the Ritsuryo system from 708 (Wadō 1st year) to 963 (Ōwa 3rd year).
[1] All of the included coins are circular in shape, have a square hole in the center, and were worth 1 mon.
[2][3] It was first minted in 708 CE on order of Empress Genmei, Japan's 43rd Imperial ruler.
[2] "Wadōkaichin" is the reading of the four characters printed on the coin, and is thought to be composed of the era name Wadō (和銅, "Japanese copper"), which could alternatively mean "happiness", and "Kaichin", thought to be related to "Currency".
The Kōchōsen Japanese system of coinage became strongly debased, with its metallic content and value decreasing.