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Early life

William was born in 1027 or 1028 at Falaise, Duchy of Normandy, most likely towards the end of 1028. He was the only son of Robert I, son of Richard II. His mother, Herleva, was a daughter of Fulbert of Falaise, who may have been a tanner or embalmer. Herleva was possibly a member of the ducal household, but did not marry Robert. She later married Herluin de Conteville, with whom she had two sons – Odo of Bayeux and Count Robert of Mortain – and a daughter whose name is unknown. One of Herleva's brothers, Walter, became a supporter and protector of William during his minority. Robert I also had a daughter, Adelaide, by another mistress.

Robert I succeeded his elder brother Richard III as duke on 6 August 1027. The brothers had been at odds over the succession, and Richard's death was sudden. Robert was accused by some writers of killing Richard, a plausible but now unprovable charge. Conditions in Normandy were unsettled, as noble families despoiled the Church and Alan III of Brittany waged war against the duchy, possibly in an attempt to take control. By 1031 Robert had gathered considerable support from noblemen many of whom would become prominent during William's life. They included the duke's uncle Robert, the archbishop of Rouen, who had originally opposed the duke; Osbern, a nephew of Gunnor the wife of Richard I; and Gilbert of Brionne, a grandson of Richard I. After his accession, Robert continued Norman support for the English princes Edward and Alfred, who were still in exile in northern France.

Robert may have been briefly betrothed to a daughter of King Cnut, but no marriage took place. It is unclear whether William would have been supplanted in the ducal succession if Robert had had a legitimate son. Earlier dukes had been illegitimate, and William's association with his father on ducal charters appears to indicate that William was considered Robert's most likely heir. In 1034 the duke decided to go on pilgrimage to Jerusalem. Although some of his supporters tried to dissuade him, he convened a council in January 1035 and had the assembled Norman magnates swear fealty to William as his heir before leaving for Jerusalem. He died in early July at Nicea, on his way back to Normandy.