William of Hauteville ( 1027 – 1080) was one of the younger sons of Tancred of Hauteville by his second wife Fressenda. He is usually called Willermus instead of Wilelmus in Latin annals and so is often called Guillerm instead of Guillaume in French. Life William left Normandy around 1053 with his elder half-brother Geoffrey and full brother Mauger. He participated in the Battle of Civitate in the years of his arrival and was received cordially by his half-brother Humphrey, the reigning count of Apulia. In 1055, he distinguished himself in the taking of the castle of San Nicandro (today San Licandro) close to Eboli, which formed the nucleus of his county of the Principato, with which he was invested by Humphrey in 1055. In 1058, he married Maria, the daughter of Guy, Duke of Sorrento and brother of Guaimar IV of Salerno. He inherited all Guy's lands in the principality of Salerno and fought with Guaimar's successor, Gisulf II, whose lands he ate away at until little was left but Salerno itself. He also inherited the Capitanate from Mauger, who died between 1054 and 1060. That last possession he gave to Geoffrey, out of fraternal love, Malaterra informs us.

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William of Hauteville ( 1027 – 1080) was one of the younger sons of Tancred of Hauteville by his second wife Fressenda. He is usually called Willermus instead of Wilelmus in Latin annals and so is often called Guillerm instead of Guillaume in French.