[[Image:Guinea 1610342.jpg|thumb|George III, 1775 guinea]]
[[Image:Guinea Spade 692183.jpg|thumb|George III, spade guinea, 1795]]
The guineas of [[George III of the United Kingdom|King George III]] weighed {{cvt|8.4|g|ozozt|2}} and were {{cvt|25|mm|in|2}} in diameter, with an average gold purity (at the time of the 1773 assay) of 0.9146 (meaning it contained {{cvt|7.7|g|oz|2}} of gold). They were issued with six different obverses and three reverses in 1761, 1763–79, 1781–99, and 1813. All the obverses show right-facing busts of the king with the legend {{sc|georgivs iii dei gratia}} with different portraits of the king. The reverse of guineas issued between 1761 and 1786 show a crowned shield bearing the arms of England+Scotland, France, Ireland and Hanover, with the legend {{sc|m b f et h rex f d b et l d s r i a t et e}} and the date ("King of Great Britain, France and Ireland, Defender of the Faith, Duke of Brunswick and Lüneburg, Arch-Treasurer and Elector of the Holy Roman Empire"). In 1787 a new design of reverse featuring a spade-shaped shield was introduced, with the same legend; this has become known as the ''spade guinea''.
In 1774 almost 20 million worn guineas of King William III and Queen Anne were melted down and recoined as guineas and half-guineas.
