Although the coin is no longer circulated, the term guinea survives in some circles, notably horse racing and in the sale of rams, to mean an amount of one pound and five pence in decimalised currency (equivalent to 21 shillings).
The obverse and reverse of this coin were designed by John Roettier (1631–c. 1700). The obverse showed a fine right-facing bust of the king wearing a laurel wreath (amended several times during the reign), surrounded by the legend CAROLVS II DEI GRATIA ("Charles II by the grace of God"), while the reverse showed four crowned cruciform shields bearing the arms of England, Scotland, France, and Ireland, between which were four sceptres, and in the centre were four interlinked "C"s, surrounded by the inscription MAG BR FRA ET HIB REX date ("Of Great Britain, France, and Ireland King"). To avoid confusion with gilded silver coins the edge was milled to deter clipping or filing (and to distinguish it from the silver half-crown which had edge lettering). Until 1669 the milling was perpendicular to the edge, giving vertical grooves, while from 1670 the milling was diagonal to the edge.
John Roettier continued to design the dies for this denomination in the reign of James II. In this reign, the coins weighed 8.5 g with a diameter of 25–26 mm, and were minted in all years between 1685 and 1688, with an average gold content of 0.9094. Coins of each year were issued both with and without the elephant and castle mark. The king's head faces left in this reign, and is surrounded by the inscription IACOBUS II DEI GRATIA ("James II by the grace of God"), while the obverse is the same as in Charles II's reign except for omitting the interlinked "C"s in the centre of the coin. The edge of the coins are milled diagonally.
With the removal of James II in the Glorious Revolution of 1688, his daughter Mary, and her husband Prince William of Orange ruled jointly by agreement as co-monarchs. Their heads appear conjoined on the guinea piece in Roman style, with William's head uppermost, with the legend GVLIELMVS ET MARIA DEI GRATIA. In a departure from the previous reigns the reverse featured a totally new design of a large crowned shield which bore the arms of imperial France in the first quarter, of Scotland in the second quarter, of Ireland in the third quarter, and of England in the fourth quarter, the whole ensemble having a small shield in the centre bearing the rampant lion of Nassau; the legend on the obverse read MAG BR FR ET HIB REX ET REGINA date. By the early part of this reign the value of the guinea had increased to nearly thirty shillings. The guineas of this reign weighed 8.5 grams, were 25–26 mm in diameter, and were the work of James and Norbert Roettier and were produced in all years between 1689 and 1694 both with and without the elephant and castle; in 1692 and 1693 the mark of the elephant alone was also used.
Following the death of Queen Mary from smallpox in 1694, William continued to reign as William III. The guinea coin was produced in all years from 1695 to 1701, both with and without the elephant and castle, the design probably being the work of Johann Crocker, also known as John Croker, since James Roettier had died in 1698 and his brother Norbert had moved to France in 1695.
The coins of William III's reign weighed 8.4 g with an average gold content of 0.9123. The diameter was 25–26 mm until 1700 and 26–27 mm in 1701. William's head faces right on his coins, with the legend GVLIELMVS III DEI GRATIA, while the reverse design of William and Mary's reign was judged to be unsuccessful, so the design reverted to that used by Charles II and James II, but with a small shield with the lion of Nassau in the centre, with the legend MAG BR FRA ET HIB REX date. The coin had a diagonal milled edge.
The reign of Queen Anne (1702-1714) produced guineas in all years between 1702 and 1714 except for 1704. The 1703 guinea bears the word VIGO under the Queen's bust, to commemorate the origin of the gold taken from the Spanish ships at the Battle of Vigo Bay.
With the Acts of Union 1707 creating a unified Kingdom of Great Britain through the union of the Parliament of Scotland with the Parliament of England, the design of the reverse of the first truly British guinea was changed. Until the Union, the cruciform shields on the reverse showed the arms of England, Scotland, France, and Ireland in order, separated by sceptres and with a central rose, and the legend MAG BRI FR ET HIB REG date ("Of Great Britain, France, and Ireland Queen"). With the Act of Union, the English and Scottish arms appear conjoined on one shield, with the left half being the English arms and the right half being the Scottish arms, and the order of arms appearing on the shields becomes England and Scotland, France, England and Scotland, Ireland. The elephant and castle can appear on the coins of 1708 and 1709. The centre of the reverse design shows the Star of the Order of the Garter. The coins weighed 8.3 g, were 25 mm in diameter, and had a gold content of 0.9134. The edge of the coin is milled diagonally.
George I's guinea coins were struck in all years between 1714 and 1727, with the elephant and castle sometimes appearing in 1721, 1722, and 1726. His guineas are notable for using five different portraits of the king, and the 1714 coin is notable for declaring him to be Prince Elector of the Holy Roman Empire. The coins weighed 8.3-8.4 grams, were 25-26 millimetres in diameter, and the average gold content was 0.9135.
The 1714 obverse shows the right-facing portrait of the king with the legend GEORGIVS D G MAG BR FR ET HIB REX F D ("George, by the grace of God of Great Britain, France, and Hibernia King, Fidei Defensor"), while the later coins bear the legend GEORGIVS D G M BR FR ET HIB REX F D. The reverse follows the same general design as before, except the order of the shields is England and Scotland, France, Ireland, and Hanover, with the legend in 1714 BRVN ET LVN DVX S R I A TH ET PR EL 1714 ("Duke of Brunswick and Lueneburg, Arch-Treasurer and Prince Elector of the Holy Roman Empire"), and in other years BRVN ET L DVX S R I A TH ET EL date ("Duke of Brunswick and Lueneburg, Arch-Treasurer and Elector of the Holy Roman Empire"). The edge of the coin is milled diagonally.
The value of the guinea had fluctuated over the years from twenty to thirty shillings, and back down to twenty-one shillings and sixpence by the start of George's reign. A Royal Proclamation of December 1717 fixed the value of the guinea at twenty-one shillings.
George II's guinea pieces are a complex issue, with eight obverses and five reverses used through the 33 years of the reign. The coins were produced in all years of the reign except 1742, 1744, 1754, and 1757. The coins weighed 8.3–8.4 g, and were 25–26 mm in diameter except for some of the 1727 coins which were 24–25 mm. The average gold content was 0.9140. Some coins issued between 1729 and 1739 carry the mark EIC under the king's head, to indicate the gold was provided by the East India Company, while some 1745 coins carry the mark LIMA to indicate the gold came from Admiral George Anson's round-the-world voyage. In the early part of the reign the edge of the coin was milled diagonally, but from 1739 following the activities of a particularly bold gang of guinea filers for whom a reward was posted, the milling was changed to produce the shape of a chevron or arrow-head. In 1732 the old hammered gold coinage was demonetised, and it is thought that some of the old coins were melted down to create more guineas.
The obverse has a left-facing bust of the king with the legend GEORGIVS II DEI GRATIA (GEORGIUS II DEI GRATIA between 1739 and 1743), while the reverse features a single large crowned shield with the quarters containing the arms of England+Scotland, France, Hanover, and Ireland, and the legend M B F ET H REX F D B ET L D S R I A T ET E ("King of Great Britain, France and Ireland, Defender of the Faith, Duke of Brunswick and Lueneburg, Arch-Treasurer and Elector").
The guineas of George III weighed 8.4 g and were 24 mm in diameter, with an average gold content (at the time of the 1773 assay) of 0.9146. They were issued with six different obverses and three reverses in 1761, 1763–1779, 1781–1799, and 1813. All the obverses show right-facing busts of the king with the legend 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 M B F ET H REX F D B ET L D S R I A T ET E date ("King of Great Britain, France and Ireland, Defender of the Faith, Duke of Brunswick and Lueneburg, 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 William III and Queen Anne were melted down and recoined as guineas and half-guineas.
Towards the end of the century gold began to become scarce and rise in value. The French Revolution and the subsequent French Revolutionary Wars had drained gold reserves and people started hoarding coins. Parliament passed a law making banknotes legal tender in any amount, and in 1799 the production of guineas was halted, although half and third-guineas continued to be struck. Following the Act of Union between Great Britain and Ireland in 1800, the king's titles changed, and an Order in Council of 5 November 1800 directed the Master of the Mint to prepare a new coinage, but although designs were prepared, the production of guineas was not authorised.
In 1813 it was necessary to strike 80,000 guineas to pay the Duke of Wellington's army in the Pyrenees, as the local people would accept only gold in payment. This issue has become known as the Military Guinea. At this time gold was still scarce, and the guinea was trading on the open market for 27 shillings in paper money, so the coining of this issue for the army's special needs was a poor deal for the government, and this was the last issue of guineas to be minted. The reverse of the military guinea is a unique design, showing a crowned shield within a Garter, with HONI SOIT QUI MAL Y PENSE on the Garter, and BRITANNIARUM REX FIDEI DEFENSOR ("King of Britain, Defender of the Faith") around the edge, and "1813" between the edge inscription and the garter.
In the Great Recoinage of 1816, the guinea was replaced as the major unit of currency by the pound, with a coin called a sovereign.
Even after the coin ceased to circulate, the name guinea was long used to indicate the amount of 21 shillings (£1.05 in decimalised currency). The guinea had an aristocratic overtone; professional fees and payment for land, horses and art were often quoted in guineas until decimalisation in 1971. It is still quoted in the pricing and sale of livestock at auction, racehorses, and many major horse races in both Great Britain, Ireland, and Australia bear names ending in "1,000 Guineas" or "2,000 Guineas," even though the values of their purses today are much higher.