Seigniorage revenues are the profits generated when monetary authorities issue currency. The privilege of minting official coins developed into the concept of seigniorage. Seigniorage is the net revenue derived from the issuing of currency. Economists regard seigniorage as a form of inflation tax, returning resources to the currency issuer. Historically, seigniorage was the profit resulting from producing coins. Successful coin types of high nobility would be copied by lower nobility for seigniorage. Seigniorage is the positive return, or carry, on issued notes and coins (money in circulation). Over half of Zimbabwe's government revenue in 2008 was reportedly seigniorage. This has also been called a citizens' dividend or a distribution of future seigniorage. Seigniorage can be a convenient source of revenue for a government. The price spread between face value and commodity value is called seigniorage. The difference in these values is seigniorage.