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The de Clare family of Norman lords were associated with the Welsh Marches, Suffolk, Surrey, Kent (especially Tonbridge) and Ireland. They were descended from Richard fitz Gilbert, who accompanied William the Conqueror into England during the Norman conquest of England.In the paternal line they were illegitimate descendants of the House of Normandy, through one of Richard I, Duke of Normandy's sons[a] Geoffrey, Count of Eu.
The Clare family descends from Gilbert Crispin, Count of Brionne and Eu, whose father Godfrey was the eldest of the illegitimate sons of Richard I, Duke of Normandy. Gilbert was one of the guardians of William II, who became Duke of Normandy as a child in 1035. When Gilbert was assassinated in 1039 or 1040, his young sons Baldwin de Meules et du Sap and Richard de Bienfaite et d'Orbec fled with their guardians to Baldwin V, Count of Flanders; they returned to Normandy when William married Baldwin's daughter in 1053, and William took them into high favour.
After the conquest of England Richard received huge estates including Clare and Tonbridge, the estate whose name was normally coupled with his. According to Richard Mortimer, he was "the founder of the English, Welsh, and Irish baronial family which historians usually call 'of Clare'."
Historical sources are vague and sometimes contradictory about when the name de Clare came into common usage, but Richard fitz Gilbert (of Tonbridge) is once referred to as Richard of Clare in the Suffolk return of the Domesday Survey. Baldwin de Meules was left in charge of Exeter on its submission (1068) and made sheriff of Devonshire. Large estates in Devonshire and Somersetshire are entered to him in Domesday as "Baldwin of Exeter" or "Baldwin the Sheriff". 
On his death, Richard's English estates passed to his son Gilbert Fitz Richard (died 1114/7). Gilbert's eldest son Richard (died ca. 1136) was the ancestor of the earls of Hertford and Gloucester. Gilbert's younger son Gilbert, establishing himself in Wales, acquired the earldom of Pembroke or of Striguil. The elder line obtained (probably from King Stephen) the earldom of Hertford, and were thenceforth known as earls of Hertford or of Clare.
John Horace Round suggested that it was probably because
[Gilbert] and the Clares had no interests in Hertfordshire that they were loosely and usually styled the earls of (de) Clare.
In the Dictionary of National Biography he stated that investigation showed that the claim that they were "styled earls of Clare" before they were earls of Hertford was not true; they were alternately called Hertford or of Clare. On the other hand, Frank Barlow places Gilbert de Clare as Earl of Hertford in the group of barons given earldoms between 1138 and 1142, and states that they all had "substantial local interests". Ralph Henry Carless Davis states that Gilbert was a witness as Earl of Hertford at Christmas 1141, and it is generally believed that he had been Earl since 1138; but that there is no prospect of clarifying the matter because of the others of the same name. He notes also that "In a military capacity earls figure largely in the capacity of defenders of their counties in the chronicles of Stephen's reign." He therefore argues against the title as a personal dignity at that period. The general scholarly view is now that the title earl of Clare was self-assumed.
In 1217–20 Gilbert de Clare, earl of Hertford or Clare (died 1230), inherited the estates of William Fitz Robert, 2nd Earl of Gloucester (died 1183), including the earldom and honour of Gloucester and the lordship of Glamorgan. Richard de Clare, 2nd Earl of Pembroke (died 1176), known as Strongbow, had no sons and with his death this line came to an end, his many Irish and Welsh possessions passing to his daughter Isabel, who married William Marshal, (c. 1146 – 14 May 1219) who then became known as William Marshal, 1st Earl of Pembroke.
The coat and shield with the three chevrons was probably first used at the end of the 12th century. The stained glass window above is not earlier than Gilbert I (died 1230), the first de Clare lord to be buried in the chancel of Tewkesbury Abbey. The three chevrons can be seen in the shield of the old county of Glamorgan and in those of many in modern-day Welsh local authorities and bodies such as Cardiff University and Pontypridd Rugby Football Club, and also in one half of the shield of Clare College, Cambridge.
Origins and family ties with notable members of Clare.
Richard I, Duke of Normandy († 996) │ └>Geoffrey, Count of Eu († 1015) │ └>Gilbert, Count of Brionne († 1040), Count of Eu │ ├─>Baldwin fitz Gilbert († 1090), Lord of Sap and Meules, │ feudal baron of Okehampton, Sheriff of Devon.
[ de Clare Family ]
│ └─>Richard fitz Gilbert († 1090), Lord of Bienfaite and of Orbec, │ also Lord of Clare and of Tonbridge, Joint Chief Justiciar of England │ ├─>Roger († 1131), Lord of Bienfaite and of Orbec │ ├─>Robert († 1134), Lord of Little Dunmow (Essex) │ ├─>Richard, Abbot of Ely │ ├─>Walter de Clare († 1138), Lord of Netherwent │ └─>Gilbert († 1117), Lord of Clare, Tonbridge and Cardigan (See Table 2) │ ├─>Richard († 1136), sometimes designated as Earl of Hertford │ ├─>Gilbert († 1148/49), 1st Earl of Pembroke, │ successor to his uncles Roger and Walter │ └─>Baldwin († 1154/1166), Lord of Bourne (see Baldwin of Clare)
Gilbert († 1117), Lord of Clare and of Tonbridge and Cardigan │ ├─>Richard († 1136), Lord of Clare, Tonbridge and Cardigan │ │ │ ├─>Gilbert († 1152), 1st Earl of Hertford │ │ │ ├─>Roger sometimes called the good Earl of Hertford († 1173), 2nd Earl of Hertford │ │ │ │ │ ├─>Richard († 1217), 3rd Earl of Hertford and 1st of Gloucester │ │ │ Amicie de Gloucester, countess de jure of Gloucester │ │ │ │ │ │ │ └─> Table 3 │ │ │ │ │ └─>Aveline († 1164), married Geoffrey fitz Piers, 1st Earl of Essex │ │ │ ├─>Rohaise, married Gilbert de Gant, Earl of Lincoln │ │ │ ├─>Alice, married Cadwaladr ap Gruffydd, prince of Gwynedd │ │ │ └─>Lucy, married Baldwin de Redvers, 1st Earl of Devon │ ├─>Gilbert († 1148/49), 1st Earl of Pembroke (1138) │ │ x Isabelle de Beaumont, │ │ daughter of Robert de Beaumont, 1st Earl of Leicester │ │ │ └─>Richard also called Strongbow († 1176), 2nd Earl of Pembroke │ │ attempted to take control of Ireland │ │ │ ├─>Gilbert of Striguil (1173–1185), 3rd Earl of Pembroke │ │ │ └─>Isabelle († 1220) │ x William Marshal became the first Earl of Pembroke │ This branch of the family being extinguished. │ └─>Baldwin († 1154), Lord of Bourne (see Baldwin of Clare)
Richard († 1217), 3rd Earl of Hertford and Gloucester │ Amicie of Gloucester, Countess de jure of Gloucester │ ├─>Gilbert († 1230), 4th Hertford and 5th Earl of Gloucester │ │ x Isabelle, daughter of William Marshal, 1st Earl of Pembroke, │ │ heir of the younger branch of Clare (see table 2) │ │ │ │ │ ├─>Richard († 1262), 5th Earl of Hertford, 6th Earl of Gloucester │ │ │ │ │ ├─>Gilbert also known as the Red Earl († 1295), 6th Earl of Hertford, 7th Earl of Gloucester │ │ │ │ Co-guardian of England in the death of King Henry III │ │ │ │ │ │ │ └─>Gilbert († 1314), 7th Hertford and 8th Earl of Gloucester │ │ │ │ │ │ │ │ ├─>Bogo (1248–1294), member of the clergy │ │ │ │ │ └─>Thomas († 1287), Lord of Thomond, Chancellor of Ireland │ │ │ └─>Isabelle († 1264), married Robert de Brus, 5th Lord of Annandale │ │ │ └─> Kings of Scotland of the House of Bruce. │ └─>Mathilde, married Rhys ap Rhys († 1234), prince of Deheubarth