William de Percy, 1st Baron Percy

William I (Guillaume) de Percy (d.1096/9), 1st feudal baron of Topcliffe in North Yorkshire, known as Guillaume aux grenons (or gernons, "with whiskers", later forming the first name Algernon, frequently used by the Percy family), was a Norman nobleman who arrived in England immediately after the Norman Conquest of 1066. He was the founder (via an early 13th century female line) of the powerful English House of Percy, Earls of Northumberland, and (via an 18th century female line) Dukes of Northumberland, a great historical House of England "that, like Caesar's, has been artificially preserved (twice) to the present time". The male line ended in 1174/5 on the death without male progeny of his grandson William II de Percy, but the surname "Percy" was re-adopted by the latter's younger grandson Richard de Louvain (d.1244), whose own "Percy" descendants again failed in the male line in 1670 on the death of Joceline Percy, 11th Earl of Northumberland, and was again re-adopted by the latter's great-grand-daughter's husband Sir Hugh Smithson, 4th Baronet (c.1714-1786), created Duke of Northumberland, whose descendants survive today.

The Cartulary of Whitby Abbey states that Hugh d'Avranches (later 1st Earl of Chester) and William de Percy arrived in England in 1067, one year after the Norman Conquest.

It is possible that Percy had been one of the Normans to whom King Edward the Confessor had given lands, but who were later expelled by King Harold (d.1066). This may explain Percy's unusual Norman epithet, Aux gernons ("Bewhiskered"), as the Normans were generally clean-shaven, unlike the English, and possibly Percy had assimilated the local custom. Later generations of Percys would use the sobriquet in the form of the first name "Algernon".

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