Question: The 1st Earl Spencer was the great grandson of what duke?

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James Butler, 1st Duke of Ormond: Lieut.-Gen.  Sir James FitzThomas Butler, 1st Duke of Ormond, 1st Marquess of Ormond, 12th Earl of Ormond, 5th Earl of Ossory, 4th Viscount Thurles, 1st Baron Butler of Llanthony, 1st Earl of Brecknock KG, PC (19 October 1610 – 21 July 1688) was an Anglo-Irish statesman and soldier, known as Earl of Ormond from 1634 to 1642 and Marquess of Ormond from 1642 to 1661.  Following the failure of the senior line of the Butler family, he was the second of the Kilcash branch to inherit the earldom.  His friend, the 1st Earl of Strafford, caused him to be appointed the commander of the Cavalier forces in Ireland.  From 1641 to 1647, he led the fighting against the Irish Catholic Confederation.  From 1649 to 1650 he was the leading commander of the Royalist forces in the fight against the Cromwellian conquest of Ireland.  In the 1650s he lived in exile in Europe with King Charles II of England.  Upon the restoration of Charles to the throne in 1660, Ormonde became a major figure in English and Irish politics, holding many high government offices.

Lord Dingwall: Lord Dingwall is a title in the Peerage of Scotland.  It was created in 1609 for Sir Richard Preston, with remainder to his heirs whatsoever.  In 1619 he was further honoured when he was made Baron Dunmore and Earl of Desmond in the Peerage of Ireland, with remainder to heirs male.  On his death in 1628 the Irish titles became extinct while he was succeeded in the Scottish lordship by his daughter Elizabeth, the second Lady Dingwall.  She was the wife of James Butler, 1st Duke of Ormonde.  Their eldest son Thomas Butler, Earl of Ossory, was summoned by writ to the English Parliament as Baron Butler, of Moore Park, in 1666.  However, he predeceased his parents who were both succeeded by their grandson, the second Duke and third Lord Dingwall.  He had already succeeded his father as second Baron Butler.  However, the Duke was attainted in 1715 and his titles forfeited.  In 1871, Francis Cowper, 7th Earl Cowper, managed to obtain a reversal of the attainder of the lordship of Dingwall and barony of Butler and became the fourth Lord Dingwall and third Baron Butler.  He was the great-great-great-grandson of Henrietta Butler, Countess of Grantham (wife of Henry de Nassau d'Auverquerque, 1st Earl of Grantham), second daughter of Thomas Butler, Earl of Ossory and 1st Baron Butler, whose second daughter Lady Henrietta de Nassau d'Auverquerque married William Clavering-Cowper, 2nd Earl Cowper.  In 1880 he also succeeded his mother as eighth Baron Lucas of Crudwell.  For later history of the lordship of Dingwall and barony of Butler, see the Baron Lucas of Crudwell.

Earl of Berkshire: Earl of Berkshire is a title that has been created twice in the Peerage of England.  It was created for the first time in 1621 for Francis Norris, 1st Earl of Berkshire.  For more information on this creation (which became extinct on his death in 1622), see the Earl of Abingdon and also the Earl of Lindsey.  The second creation came in 1626 in favour of Thomas Howard, 1st Viscount Andover.  He was the second son of Thomas Howard, 1st Earl of Suffolk, second son of the second marriage of Thomas Howard, 4th Duke of Norfolk.  His mother was Katherine, daughter of Sir Henry Knyvett of Charlton in Wiltshire.  Howard had already been created Baron Howard of Charlton, in the County of Wiltshire, and Viscount Andover, in the County of Southampton, in 1622.  These titles are also in the Peerage of England.  Lord Berkshire succeeded to the Charlton estate through his mother in 1638.  He was succeeded by his eldest son, the second Earl.  He had already in 1640 been summoned to the House of Lords through a writ of acceleration in his father's junior title of Baron Howard of Charlton.  He had no sons and on his death in 1679 the titles passed to his younger brother, the third Earl.  He represented Wallingford in the House of Commons.  He also died without male issue and was succeeded by his great-nephew, the fourth Earl.  He was the grandson of the Hon. William Howard, fourth son of the first Earl.  In 1745 he succeeded his third cousin as eleventh Earl of Suffolk.  For further history of the titles, see the Earl of Suffolk.

Hugh le Despencer, Baron le Despencer (1338): Hugh le Despencer, 2nd Baron le Despencer (1308–1349), Lord of Glamorgan, was the eldest son and heir of Hugh Despenser the Younger and grandson of Hugh le Despenser, 1st Earl of Winchester.  His father and grandfather were both executed in 1326.  His mother was Eleanor de Clare, "suo jure" Lord of Glamorgan, daughter of Gilbert de Clare, 6th Earl of Hertford and Joan of Acre.  Through his mother, Hugh was a great grandson of King Edward I.  He married Elizabeth Montague (d.1359), and both are buried in Tewkesbury Abbey, Gloucestershire, which abbey was founded by Robert Fitzhamon (d.1107), the first Norman feudal baron of Gloucester and Lord of Glamorgan.

Hugh de Audley, 1st Earl of Gloucester: Hugh de Audley, 1st Baron Audley and 1st Earl of Gloucester (3rd Creation) (c. 1291 – 10 November 1347) was the second son of Sir Hugh de Audley, Lord Audley by Iseult de Mortimer and Great great grandson of King Henry II.  He held many offices including Knight of Stratton in Stratton Audley, Oxfordshire, of Gratton, Staffordshire, the King's bachelor, Sheriff of Rutland, and was the English Ambassador to France in 1341.

Henrietta Ponsonby, Countess of Bessborough: Henrietta Ponsonby, Countess of Bessborough (16 June 1761 – 11 November 1821), born Lady Henrietta Frances Spencer (generally called Harriet), was the wife of Frederick Ponsonby, 3rd Earl of Bessborough, and mother of the notorious Lady Caroline Lamb.  Her father, John Spencer, 1st Earl Spencer, was a great-grandson of John Churchill, 1st Duke of Marlborough.  Her sister was Georgiana Cavendish, Duchess of Devonshire.

John Spencer, 1st Earl Spencer: John Spencer, 1st Earl Spencer (19 December 1734 – 31 October 1783) was a British peer and politician.

Earl Spencer (peerage): Earl Spencer is a title in the Peerage of Great Britain that was created on 1 November 1765, along with the title Viscount Althorp, of Althorp in the County of Northampton, for John Spencer, 1st Viscount Spencer.  He was a member of the prominent Spencer family and a great-grandson of the 1st Duke of Marlborough.  Previously, he had been created Viscount Spencer, of Althorp in the County of Northampton, and Baron Spencer of Althorp, of Althorp in the County of Northampton, on 3 April 1761.

Henry Howard, 10th Earl of Suffolk: Henry Howard, 10th Earl of Suffolk (1 January 1706 – 22 April 1745) was the only child of Henrietta Howard, Countess of Suffolk and Charles Howard, 9th Earl of Suffolk.  He was styled Lord Walden from 1731 to 1733.  He married Sarah Inwen, daughter of Thomas Inwen and Sarah Hucks, on 13 May 1735, but died on 22 April 1745 at age 39 with no surviving issue and no brothers to whom the title could be passed.  The line of Earls of Suffolk back to Henry's great-great grandfather Thomas Howard, 1st Earl of Suffolk thus died out, and the title passed to a great grandson of the 1st Earl (via the 1st Earl's second son Thomas), Henry Howard, 4th Earl of Berkshire.

Conan IV, Duke of Brittany: Conan IV of Penthièvre (1138 – February 20, 1171), ("Breton: Konan IV Penteur, and Konan Breizh") called "the Young", was duke of Brittany, from 1156 to 1166.  He was the son of Bertha, Duchess of Brittany, and her first husband, Alan, 1st Earl of Richmond.  Conan IV was his father's heir as 2nd Earl of Richmond and his mother's heir as Duke of Brittany.  From his father's side, Conan was great-great-grandson of duke Geoffrey I and great grandson of Odo of Brittany.  Conan and his daughter Constance would be the only representatives of the Penthièvre dynasty of Brittany to hold the Duchy's crown.

Supporting paragraphs: Earl Spencer (peerage) John Spencer, 1st Earl Spencer
Answer: 1st Duke of Marlborough