Jocelin, Count of Louvain, came to England and married Agnes de Percy, heiress of the Percy family. Their children took their mother's name, and their descendants include the medieval Earls of Northumberland.
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Joscelin of Louvain (1121–1180) was a Brabantian nobleman who married an English heiress, Agnes de Percy, and settled in England. He took the name Percy.
Joscelin was given lands at Petworth, by William d'Aubigny, 1st Earl of Arundel. William d'Aubigny had married Adeliza of Louvain, Joscelin’s half-sister, and widow of Henry I of England.
He was a son of Godfrey I of Leuven either of Godfrey's second wife Clementia of Burgundy or of a mistress of his father.
Joscelin and Agnes had at least seven children:
- Henry de Percy
- Richard de Percy (died 1244), who was a Magna Carta surety
- Joscelin
- Radulph, went to France
- Eleanor
- Maud (born c. 1164, date of death unknown), married John de Daiville
- Lucy
The Percy estate was divided between William, son of Henry, and his brother Richard.
Retrieved from http://en.wikipedia.org/wiki/Joceline_of_Louvain