On 22 Jun 1610 [his father] William Seymour 2nd Duke Somerset (age 22) and Arabella Stewart (age 35) were married in secret at Palace of Placentia, Greenwich [Map]. For having married without permission King James I of England and Ireland and VI of Scotland (age 44) had Arabella Stewart (age 35) imprisoned in Sir Thomas Perry's House Lambeth, Surrey and he in the Tower of London [Map]. She the daughter of Charles Stewart 5th Earl Lennox 1557-1576 and Elizabeth Cavendish Countess Lennox 1555-1582. They were third cousin once removed. He a great x 3 grandson of King Henry VII of England and Ireland. She a great x 2 granddaughter of King Henry VII of England and Ireland. 
On 03 Mar 1617 [his father] William Seymour 2nd Duke Somerset (age 29) and [his mother] Frances Devereux Duchess of Somerset (age 17) were married at Drayton Bassett, Staffordshire [Map]. She the daughter of Robert Devereux 2nd Earl Essex and Frances Walsingham Countess Essex (age 50). They were fourth cousins. He a great x 3 grandson of King Henry VII of England and Ireland. 
In 1641 [his father] William Seymour 2nd Duke Somerset (age 53) was created 1st Marquess Hertford.
In or before 1645 John Seymour 4th Duke Somerset was born to William Seymour 2nd Duke Somerset (age 56) and Frances Devereux Duchess of Somerset (age 45).
In 1656 John Seymour 4th Duke Somerset (age 11) and Sarah Alston Duchess Somerset 1642-1692 (age 14) were married. He the son of William Seymour 2nd Duke Somerset (age 68) and Frances Devereux Duchess of Somerset (age 56). He a great x 4 grandson of King Henry VII of England and Ireland.
On 24 Oct 1660 [his father] William Seymour 2nd Duke Somerset (age 72) died. His grandson William Seymour 3rd Duke Somerset 1654-1671 (age 6) succeeded 3rd Duke Somerset, 2nd Marquess Hertford, 3rd Earl Hertford.
On 12 Dec 1671 William Seymour 3rd Duke Somerset 1654-1671 (age 17) died. His uncle John Seymour 4th Duke Somerset (age 26) succeeded 4th Duke Somerset, 3rd Marquess Hertford, 4th Earl Hertford. [his wife] Sarah Alston Duchess Somerset 1642-1692 (age 29) by marriage Duchess Somerset.
On 23 Apr 1674 [his mother] Frances Devereux Duchess of Somerset (age 74) died.
On 29 Apr 1675 John Seymour 4th Duke Somerset (age 30) died. He was buried at Salisbury Cathedral [Map]. His first cousin once removed Francis Seymour 5th Duke Somerset 1658-1678 (age 17) succeeded 5th Duke Somerset, 5th Earl Hertford. Marquess Hertford extinct.
On 02 Sep 1692 [his former wife] Sarah Alston Duchess Somerset 1642-1692 (age 50) died.
Paternal Family Tree: Seymour
Maternal Family Tree: Ursula St Barbe 1532-1602
Kings Wessex: Great x 18 Grand Son of King Edmund "Ironside" I of England
Kings Gwynedd: Great x 15 Grand Son of Owain "Great" King Gwynedd 1100-1170
Kings Seisyllwg: Great x 20 Grand Son of Hywel "Dda aka Good" King Seisyllwg King Deheubarth
Kings Powys: Great x 15 Grand Son of Maredudd ap Bleddyn King Powys 1047-1132
Kings England: Great x 4 Grand Son of King Henry VII of England and Ireland
Kings Scotland: Great x 13 Grand Son of William "Lion" I King Scotland
Kings Franks: Great x 14 Grand Son of Louis VII King Franks
Kings France: Great x 7 Grand Son of Charles "Beloved Mad" VI King France
Great x 4 Grandfather: John Seymour 12 x Great Grand Son of King Henry I "Beauclerc" England
Great x 3 Grandfather: John Seymour 9 x Great Grand Son of King John "Lackland" of England
Great x 4 Grandmother: Elizabeth Darell 8 x Great Grand Daughter of King John "Lackland" of England
Great x 2 Grandfather: Edward Seymour 1st Duke Somerset 6 x Great Grand Son of King Edward III of England
Great x 4 Grandfather: Henry Wentworth 4th Baron Despencer
4 x Great Grand Son of King Edward III of England
Great x 3 Grandmother: Margery Wentworth 5 x Great Grand Daughter of King Edward III of England
Great x 4 Grandmother: Anne Saye Baroness Despencer
8 x Great Grand Daughter of King John "Lackland" of England
Great x 1 Grandfather: Edward Seymour 1st Earl Hertford 5 x Great Grand Son of King Edward III of England
Great x 4 Grandfather: Thomas Stanhope
Great x 3 Grandfather: Edward Stanhope
Great x 4 Grandmother: Margaret or Mary Jerningham
Great x 2 Grandmother: Anne Stanhope Duchess Somerset 4 x Great Grand Daughter of King Edward III of England
Great x 4 Grandfather: Fulk Bourchier 10th Baron Fitzwarin
2 x Great Grand Son of King Edward III of England
Great x 3 Grandmother: Elizabeth Bourchier 3 x Great Grand Daughter of King Edward III of England
GrandFather: Edward Seymour 2 x Great Grand Son of King Henry VII of England and Ireland
Great x 4 Grandfather: Thomas Grey 1st Marquess Dorset
6 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandfather: Thomas Grey 2nd Marquess Dorset 4 x Great Grand Son of King Edward III of England
Great x 4 Grandmother: Cecily Bonville Marchioness Dorset
3 x Great Grand Daughter of King Edward III of England
Great x 2 Grandfather: Henry Grey 1st Duke of Suffolk 5 x Great Grand Son of King Edward III of England
Great x 4 Grandfather: Robert Wotton of Boughton Malherbe
Great x 3 Grandmother: Margaret Wotton Marchioness Dorset
Great x 1 Grandmother: Catherine Grey Countess Hertford Great Grand Daughter of King Henry VII of England and Ireland
Great x 4 Grandfather: William Brandon
5 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandfather: Charles Brandon 1st Duke of Suffolk 6 x Great Grand Son of King Edward "Longshanks" I of England
Great x 4 Grandmother: Elizabeth Bruyn
Great x 2 Grandmother: Frances Brandon Duchess of Suffolk Grand Daughter of King Henry VII of England and Ireland
Great x 4 Grandfather: King Henry VII of England and Ireland
3 x Great Grand Son of King Edward III of England
Great x 3 Grandmother: Mary Tudor Queen Consort France Daughter of King Henry VII of England and Ireland
Great x 4 Grandmother: Elizabeth York Queen Consort England Daughter of King Edward IV of England
Father: William Seymour 2nd Duke Somerset 3 x Great Grand Son of King Henry VII of England and Ireland
GrandMother: Honora Rogers
John Seymour 4th Duke Somerset 1645-1675 4 x Great Grand Son of King Henry VII of England and Ireland
Great x 4 Grandfather: John Devereux 9th Baron Ferrers of Chartley 7 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandfather: Walter Devereux 1st Viscount Hereford 4 x Great Grand Son of King Edward III of England
Great x 4 Grandmother: Cecily Bourchier Baroness Ferrers Chartley 3 x Great Grand Daughter of King Edward III of England
Great x 2 Grandfather: Richard Devereux 5 x Great Grand Son of King Edward III of England
Great x 4 Grandfather: Thomas Grey 1st Marquess Dorset 6 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandmother: Mary Grey Baroness Ferrers Chartley 4 x Great Grand Daughter of King Edward III of England
Great x 4 Grandmother: Cecily Bonville Marchioness Dorset 3 x Great Grand Daughter of King Edward III of England
Great x 1 Grandfather: Walter Devereux 1st Earl Essex 6 x Great Grand Son of King Edward III of England
Great x 4 Grandfather: Edward Hastings 2nd Baron Hastings Baron Botreaux, Hungerford and Moleyns
3 x Great Grand Son of King Edward III of England
Great x 3 Grandfather: George Hastings 1st Earl Huntingdon 4 x Great Grand Son of King Edward III of England
Great x 4 Grandmother: Mary Hungerford Baroness Hastings, 4th Baroness Hungerford, 5th Baroness Botreaux and 2nd Baroness Moleyns
4 x Great Grand Daughter of King Edward III of England
Great x 2 Grandmother: Dorothy Hastings 5 x Great Grand Daughter of King Edward III of England
Great x 4 Grandfather: Henry Stafford 2nd Duke of Buckingham
3 x Great Grand Son of King Edward III of England
Great x 3 Grandmother: Anne Stafford Countess Huntingdon 4 x Great Grand Daughter of King Edward III of England
Great x 4 Grandmother: Catherine Woodville Duchess Buckingham Duchess Bedford
6 x Great Grand Daughter of King Henry III of England
GrandFather: Robert Devereux 2nd Earl Essex 7 x Great Grand Son of King Edward III of England
Great x 4 Grandfather: Robert Knollys
Great x 3 Grandfather: Robert Knollys
Great x 4 Grandmother: Elizabeth Troutbeck
Great x 2 Grandfather: Francis Knollys
Great x 3 Grandmother: Lettice Peniston
Great x 1 Grandmother: Lettice Knollys Countess Essex 6 x Great Grand Daughter of King Edward III of England
Great x 4 Grandfather: Thomas Carey
5 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandfather: William Carey 4 x Great Grand Son of King Edward III of England
Great x 4 Grandmother: Margaret Spencer 3 x Great Grand Daughter of King Edward III of England
Great x 2 Grandmother: Catherine Carey 5 x Great Grand Daughter of King Edward III of England
Great x 4 Grandfather: Thomas Boleyn 1st Earl Wiltshire and Ormonde 6 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandmother: Mary Boleyn
7 x Great Grand Daughter of King Edward "Longshanks" I of England
Great x 4 Grandmother: Elizabeth Howard Countess of Wiltshire and Ormonde
6 x Great Grand Daughter of King Edward "Longshanks" I of England
Mother: Frances Devereux Duchess of Somerset 8 x Great Grand Daughter of King Edward III of England
Great x 3 Grandfather: James Walsingham
Great x 2 Grandfather: William Walsingham
Great x 4 Grandfather: Walter Writtle
Great x 3 Grandmother: Eleanor Writtle
Great x 1 Grandfather: Francis Walsingham 8 x Great Grand Son of King Edward "Longshanks" I of England
Great x 4 Grandfather: William Denny
Great x 3 Grandfather: Edmund Denny
Great x 2 Grandmother: Joyce Denny 7 x Great Grand Daughter of King Edward "Longshanks" I of England
Great x 4 Grandfather: Robert Troutbeck 5 x Great Grand Son of King Edward "Longshanks" I of England
Great x 3 Grandmother: Mary Troutbeck 6 x Great Grand Daughter of King Edward "Longshanks" I of England
GrandMother: Frances Walsingham Countess Essex 9 x Great Grand Daughter of King Edward "Longshanks" I of England
Great x 4 Grandfather: John St Barbe
Great x 3 Grandfather: Richard St Barbe
Great x 2 Grandfather: Henry St Barbe of Ashington Somerset
Great x 1 Grandmother: Ursula St Barbe
