Background
William was the eldest son of Henry the Mild, Duke of Brunswick-Lüneburg.
William was the eldest son of Henry the Mild, Duke of Brunswick-Lüneburg.
He is counted either as William III or William IV. William retained only the western part of Wolfenbüttel (to the west of the River Leine and separated from the rest by the Bishopric of Hildesheim), which was to become known as the Principality of Calenberg. After Henry died without son in 1473, William had control of both parts of Wolfenbüttel again. In Berlin between 30 May/6 June 1423, William married firstly Cecilia (born c 1405 – died 4 January 1449), daughter of Frederick I, Elector of Brandenburg.
They had two sons:
Frederick "the Turbulent" (c 1424–1495)
William "the Younger" (c 1425–1503)
In 1466, William married secondly Matilda, daughter of Otto II, Count of Holstein-Schauenburg-Pinneburg.
She died in Neustadt am Rübenberge on 22 July 1468, two days after giving birth a short-lived son, Otto (born Neustadt am Rübenberge 20 July 1468 – died 1471).