William was an illegitimate son of Philip of Loo, who was the son of the Flemish count Robert the Frisian and younger brother of Robert II.Richard Eales, of Ypres, styled count of Flanders (d. 1164/5)" Oxford Dictionary of National Biography William's mother was a wool carder, which further diminished his status; King Louis VI of France pointed out that she never rose from that station. The exact date of his birth is unknown, and although 1090 is the commonly used date of his birth, several other dates later in the same decade are also plausible. His brother, Theobald Sorel, was likely born of another relationship of hers. His maternal origin did not prevent him from having a large influence in Flanders.
