12 Sep is in September.
Before 12 Sep 1015 Henry Reginar I Count Louvain was born to Lambert "Bearded" Reginar I Count Louvain (age 65) and Gerberga Carolingian Duchess Lower Lorraine (age 40).
On 12 Sep 1015 Lambert "Bearded" Reginar I Count Louvain (age 65) was killed by Godfrey Ardennes Count Verdun (age 50). His son Henry Reginar I Count Louvain succeeded I Count Louvain.
On 12 Sep 1213 Peter II King Aragon (age 35) died. His son James I King Aragon (age 5) succeeded I King Aragon.
On 12 Sep 1266 the garrison at Kenilworth Castle [Map] surrendered.
On 12 Sep 1309 Alfonso "Brave" IV King Portugal (age 18) and Beatrice Ivrea Queen Consort Portugal (age 16) were married. She the daughter of Sancho IV King Castile IV King Leon and Maria Molina Queen Consort Castile Queen Consort Leon. He the son of Denis I King Portugal (age 47) and Elisabeth Barcelona Queen Consort Portugal (age 38). They were half first cousin once removed. He a great x 3 grandson of King Henry "Curtmantle" II of England. She a great x 4 granddaughter of King Henry "Curtmantle" II of England. 
On 19 May 1359 , or thereabouts, a double-royal wedding celebration took place at Reading Abbey [Map] whereby two children of King Edward III of England (age 46) were married:
John of Gaunt 1st Duke Lancaster (age 19) and Blanche Plantagenet Duchess Lancaster (age 14) were married. She by marriage Countess Richmond. She the daughter of Henry of Grosmont 1st Duke Lancaster (age 49) and Isabel Beaumont Duchess Lancaster (age 39). He the son of King Edward III of England (age 46) and Philippa of Hainault Queen Consort England (age 44). They were half second cousin once removed. She a great x 2 granddaughter of King Henry III of England. 
John Hastings 2nd Earl Pembroke (age 11) and Margaret Plantagenet Countess of Pembroke (age 12) were married. At the time John Hastings 2nd Earl Pembroke (age 11) was a ward of King Edward III of England (age 55) who would enjoy the benefit of the substantial revenue of the Earldom of Pembroke until John came of age nine years later on 12 Sep 1368. She died two or so years later probably of plague. She the daughter of King Edward III of England (age 46) and Philippa of Hainault Queen Consort England (age 44). He the son of Laurence Hastings 1st Earl Pembroke and Agnes Mortimer Countess of Pembroke (age 42). They were half fourth cousins. He a great x 5 grandson of King John "Lackland" of England. 
On 12 Sep 1368 Blanche Plantagenet Duchess Lancaster (age 23) died at Tutbury Castle [Map]. Her last words were said to be "Souveyne vous de moi" ("Don't forget me") the 'S' of which was possibly subsequently represented on the Lancastrian Esses Collar. She was buried at St Paul's Cathedral [Map]. Her son Henry IV King England (age 1) succeeded 3rd Earl Derby 2C 1337 and 6th Earl Lancaster.
On 12 Sep 1415 John Mowbray 3rd Duke of Norfolk was born to John Mowbray 2nd Duke of Norfolk (age 23) and Katherine Neville Duchess Norfolk (age 15). He a great x 2 grandson of King Edward III of England.
On 12 Sep 1494 King Francis I of France was born to Charles Valois Orléans Count Angoulême (age 35) and Louise of Savoy Countess Angoulême (age 18). Coefficient of inbreeding 2.02%. 
Letters and Papers 1517. 12 Sep 1517. Giust. Desp. II. 129. 3675. SEB. GIUSTINIAN to the DOGE.
A French ambassador has arrived from the Emperor, a man of no account, apparently only to borrow money. He has not yet had an audience, either of the King, who keeps aloof at Windsor to avoid the sickness, or of Wolsey, who has gone to Walsingham. London, 12 Sept. 1517.
After 12 Sep 1570. Monument to John Shirley (deceased) at Church of St Mary and St Hardulph Breedon on the Hill, Leicestershire [Map]. Elizabethan Period. Helm with Folds. Richard and Gabriel Royley of Burton upon Trent.
John Shirley: In 1533 he was born to Francis Shirley (age 17) and Dorothy Giffard (age 42) at Staunton Harold, Leicestershire. On 12 Sep 1570 John Shirley (age 37) died at Staunton Harold, Leicestershire. He was buried at Church of St Mary and St Hardulph Breedon on the Hill, Leicestershire.
On 12 Sep 1573 Archibald Campbell 5th Earl Argyll (age 39) died. His brother Colin Campbell 6th Earl Argyll (age 29) succeeded 6th Earl Argyll.
On 12 Sep 1608 Alexander Heinrich Oldenburg I Duke Schleswig Holstein Sonderburg Augustenburg was born to Alexander Oldenburg I Duke Schleswig Holstein Sonderburg (age 35) and Dorothea Schwarzburg Duchess Schleswig Holstein Sonderburg (age 29).
Before 12 Sep 1624 Thomas Cromwell 1st Earl Ardglass (age 30) and Elizabeth Meverell Countess Ardglass were married.
On 12 Sep 1624 Wingfield Cromwell 2nd Earl Ardglass was born to Thomas Cromwell 1st Earl Ardglass (age 30) and Elizabeth Meverell Countess Ardglass.
Before 12 Sep 1631 Brien Cockayne 2nd Viscount Cullen was born to Charles Cockayne 1st Viscount Cullen (age 29) and Mary O'Brien Viscountess Cullen (age 22). On 12 Sep 1631 he was baptised.
On 12 Sep 1651 William Hamilton 2nd Duke Hamilton (age 34) died from wounds received at the Battle of Worcester. His niece Anne Hamilton 3rd Duchess Hamilton (age 19), daughter of the 1st Duke, succeeded 3rd Duchess Hamilton. Earl Cambridge 4C 1619 extinct.
On 12 Sep 1656 Philip Stanhope 1st Earl Chesterfield (age 72) died. His grandson Philip Stanhope 2nd Earl Chesterfield (age 22) succeeded 2nd Earl Chesterfield 1C 1628, 2nd Baron Stanhope of Shelford in Nottinghamshire.
On 12 Sep 1671 Archbishop William Dawes 3rd Baronet was born to John Dawes 1st Baronet and Christian Hawkins.
In 12 Sep 1683 Alfonso VI King Portugal (age 40) died. His brother Peter II King Portugal (age 34) succeeded King Portugal. Marie Françoise Élisabeth of Savoy Queen Consort of Portugal (age 37) by marriage Queen Consort Portugal for a second time albeit for three months only.
Evelyn's Diary. 12 Sep 1686. The King of Denmark (age 14) was besieging Hamburg, no doubt by the French contrivance, to embroil the Protestant Princes in a new war, that Holland, etc., being engaged, matter for new quarrel might arise: the unheard-of persecution of the poor Protestants still raging more than ever.
On 12 Sep 1691 John George Wettin III Elector Saxony (age 44) died. His son John George Wettin IV Elector Saxony (age 22) succeeded IV Elector Saxony.
After 12 Sep 1692 Charles Livingston 2nd Earl of Newburgh and Frances Brudenell Countess Newburgh were married. She by marriage Countess of Newburgh. He the son of James Livingston 1st Earl Newburgh and Catherine Howard Countess Newburgh. They were fourth cousins.
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Sherborne Abbey [Map]. On 12 Sep 1698 John Digby 3rd Earl Bristol (age 64) died without issue. Earl Bristol 1C 1622 extinct. In May 1658 Alice Bourne died. On 16 Feb 1709 Rachel Wyndham (age 64) died. William and Mary. Monument sculpted by John Nost.
Alice Bourne: She was born to Robert Bourne of Blake Hall in Essex. On 26 May 1656 John Digby 3rd Earl Bristol (age 22) and she were married. He the son of George Digby 2nd Earl Bristol (age 43) and Anne Russell Countess Bristol (age 36).
Rachel Wyndham: Around 1645 she was born to Hugh Wyndham Baron of the Exchequer (age 43) and Jane Wodehouse. After May 1658 John Digby 3rd Earl Bristol (age 24) and Rachel Wyndham (age 13) were married. He the son of George Digby 2nd Earl Bristol (age 45) and Anne Russell Countess Bristol (age 38).
On 12 Sep 1709 Charles Noel Somerset 4th Duke Beaufort was born to Henry Somerset 2nd Duke Beaufort (age 25) and Rachel Noel Duchess Beaufort (age 22). His mother Rachel Noel Duchess Beaufort (age 22) died from childbirth the next day.
On 12 Sep 1745 Francis Charteris de jure 7th Earl Wemyss (age 21) and Catherine Gordon Countess Wemyss (age 32) were married. She the daughter of Alexander Gordon 2nd Duke Gordon and Henrietta Mordaunt Duchess Gordon. He the son of James Wemyss 5th Earl Wemyss (age 46) and Janet Charteris. They were fourth cousins.
On 12 Sep 1759 Karl Anton Oldenburg I Duke Schleswig-Holstein-Sonderburg-Beck died at Szczecin. His son Friedrich Karl Ludwig Oldenburg I Duke Schleswig-Holstein-Sonderburg-Beck (age 2) succeeded Karl I Duke Schleswig-Holstein-Sonderburg-Beck.
On 12 Sep 1761 Bishop George Henry Law was born.
On 12 Sep 1769 Thomas Egerton 1st Earl Wilton (age 20) and Eleanor Assheton Viscountess Wilton (age 16) were married. She by marriage Lady Egerton and Oulton.
On 12 Sep 1779 Richard Grenville-Temple 2nd Earl Temple (age 67) died in a carriage accident without surviving issue. His nephew George Nugent Temple Grenville 1st Marquess Buckingham (age 26) succeeded 3rd Earl Temple, 4th Viscount Cobham.
On 12 Sep 1815 Charles Frederick Ashley-Cooper Ponsonby 2nd Baron de Mauley was born to William Francis Spencer Ponsonby 1st Baron de Mauley (age 28) and Barbara Ashley-Cooper Baroness Mauley.
In 12 Sep 1826 Edward Harbord 3rd Baron Suffield (age 44) and Emily Harriey Shirley Baroness Suffield were married. She by marriage Baroness Suffield.
On 12 Sep 1837 Prince Louis Hesse Darmstadt IV Grand Duke was born to Karl Wilhelm Hesse Darmstadt (age 28) and Elizabeth Prussia.
On 12 Sep 1844 James Walter Grimston 2nd Earl Verulam (age 35) and Elizabeth Joanna Weyland Countess Verulam (age 19) were married. He the son of James Walter Grimston 1st Earl Verulam (age 68) and Charlotte Jenkinson Countess Verulam.
On 12 Sep 1845 Arthur Blundell Sandys Trumbull Hill 3rd Marquess Downshire (age 56) died. His son Arthur Hill 4th Marquess Downshire (age 33) succeeded 4th Marquess Downshire. Caroline Stapleton-Cotton Marchioness Downshire (age 30) by marriage Marchioness Downshire.
On 12 Sep 1847 John Crichton-Stuart 3rd Marquess Bute was born to John Crichton-Stuart 2nd Marquess Bute (age 54) and Sophia Frederica Christina Rawdon-Hastings Marchioness Bute (age 38).
On 12 Sep 1852 Herbert Henry Asquith 1st Earl of Oxford and Asquith was born.
On 12 Sep 1859 Henry Valentine Stafford-Jerningham 9th Baron Stafford (age 57) and Emma Eliza Gerard (age 17) were married. The difference in their ages was 39 years. He a great x 4 grandson of King Charles II of England Scotland and Ireland. 
On 12 Sep 1863 Thomas Grosvenor 2nd Earl Wilton (age 63) and Susan Isabel Elton Smith Countess Wilton (age 21) were married. She by marriage Countess Wilton. The difference in their ages was 42 years. He the son of Robert Grosvenor 1st Marquess Westminster and Eleanor Egerton Marchioness Westminster.
On 12 Sep 1866 Richard Granville Hare 4th Earl of Listowel was born to William Hare 3rd Earl of Listowel (age 33).
On 12 Sep 1876 Henry Neville 3rd Marquess Abergavenny (age 22) and Violet Streatfeild (age 21) were married. He the son of William Neville 1st Marquess Abergavenny (age 49) and Caroline Vanden Bempte Johnston Marchioness Abergavenny.
On 12 Sep 1880 Arthur Claud Spencer Chichester 4th Baron Templemore was born to Arthur Henry Chichester Chichester 3rd Baron Templemore (age 26).
On 12 Sep 1892 Stanley Baldwin Prime Minister 1st Earl Baldwin (age 25) and Lucy Ridsdale Countess Baldwin (age 23) were married.
On 12 Sep 1916 Friedrich Wilhelm Hesse-Kassel (age 22) was killed.
On 12 Sep 1920 George Denison 3rd Earl of Londesborough (age 28) died unmarried. His brother Hugo Denison 4th Earl of Londesborough (age 25) succeeded 4th Earl Londesborough in Yorkshire.
On 12 Sep 1922 Wentworth Beaumont 3rd Viscount Allendale was born to Wentworth Henry Canning Beaumont 2nd Viscount Allendale (age 32) and Violet Lucy Emily Seely Viscountess Allendale (age 25).
On 12 Sep 1932 Henry FitzWalter Plumptre 20th Baron FitzWalter (age 71) died. Baron Fitzwalter 1C 1295 abeyant.
On 12 Sep 1933 William Bulkeley Barrington 9th Viscount Barrington (age 85) died. He was buried at St Andrew's Church Shrivenham.
On 12 Sep 1949 Eric Chaplin 2nd Viscount Chaplin (age 71) died. His son Anthony Chaplin 3rd Viscount Chaplin succeeded 3rd Viscount Chaplin of Saint Oswald's in Blankney in Lincolnshire. Alvilde Bridges Viscountess Chaplin (age 40) by marriage Viscountess Chaplin of Saint Oswald's in Blankney in Lincolnshire.
On 12 Sep 1953 James Albert Edward Hamilton 3rd Duke Abercorn (age 83) died. His son James Edward Hamilton 4th Duke Abercorn (age 49) succeeded 4th Duke Abercorn, 4th Marquess Hamilton. Kathleen Hamilton Duchess of Abercorn (age 48) by marriage Duchess Abercorn.
On 12 Sep 1957 Clarence Bruce 3rd Baron Aberdare (age 72) and Griselda Hervey Baroness Aberdare were married. She by marriage Baroness Aberdare of Duffryn in Glamorganshire.
On 12 Sep 1968 Nicholas Russell 6th Earl Russell was born to Conrad Russell 5th Earl Russell (age 31).
