In 1717, Newton’s mint report set the value of the gold guinea at 21 shillings.
The guinea was first minted in 1663, with much of its gold originating from the Guinea region of West Africa. It initially represented 20 shillings, or one pound, but changing gold and silver prices caused its market value to fluctuate—sometimes reaching 30 shillings.
Isaac Newton, then Master of the Mint, examined Britain’s coinage problems and reported in September 1717 that the guinea was overvalued at 21 shillings and sixpence. A royal proclamation in December 1717 fixed its official value at 21 shillings, equal to £1 1s in pre-decimal currency.
A common mix-up is the sovereign: although the sovereign later became Britain’s standard 20-shilling gold coin, it did not replace the guinea until the monetary reforms following the Great Recoinage of 1816. The word “guinea” survived afterward for professional fees, luxury goods, and horse sales.