When did the yen become the official currency of Japan?
Japanese yen
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The concept of the yen was a component of the Meiji government's modernization program of Japan's economy; which postulated the pursuit of a uniform currency throughout the country modeled after the European decimal currency system.
Before the Meiji Restoration, Japan's feudal fiefs all issued their own money, hansatsu, in an array of incompatible denominations. The New Currency Act of 1871 did away with these and established the yen, which was defined as 1.5g (0.048 troy ounces) of gold, or 24.26g (0.780 troy ounces) of silver, as the new decimal currency. The former han (fiefs) became prefectures and their mints private chartered banks, which initially retained the right to print money. To bring an end to this situation the Bank of Japan was founded in 1882 and given a monopoly on controlling the money supply.[3]