[1] He was the brother-in-law of Henry III, Holy Roman Emperor who had married his sister, Agnes de Poitou.

William VIII was one of the leaders of the allied army called to help Ramiro I of Aragon in the Siege of Barbastro (1064).

[2] This expedition was the first campaign organized by the papacy, namely Pope Alexander II,[3] against a Muslim town and stronghold in the Emirate of Zaragoza, and the precursor of the later Crusades movement.

During William VIII's rule, the alliance with the southern kingdoms of modern Spain was a political priority as shown by the marriage of all his daughters to Iberian kings.

After he divorced his first two wives, the first due to infertility, he married a third time to a much younger woman who was also his cousin Robert I of Burgundy's daughter.