Coin of account
The mill (or sometimes, mil) is a coin of account in the United States. It is equal to one-tenth of a penny, and so to one-thousandth of a dollar (= $0.001), whence the name, which means "thousandth." There was never such a coin minted by the U.S. Federal government, though some states minted these coins well into the mid-1900s. Coins of account are used in accounting and for figuring taxes, usually either property taxes or sales taxes.
From 1816 to the 1980s the British Guinea was no longer used a coin but prices for luxury items and professional services were often quoted in guineas, on the understanding that a guinea was equal to 21 shillings.