William X (1099 – 9 April 1137), called the Saint, was Duke of Aquitaine, Duke of Gascony, and Count of Poitou (as William VIII) between 1126 and 1137. He was the son of William IX by his second wife, Philippa of Toulouse. Philippa and her infant son were left in Poitiers. Long after Duke William's return, he took up with Dangerose, the wife of one of his vassals, and set aside his rightful wife, Philippa. This caused strain between father and son, until William married Aenor de Châtellerault, daughter of his father's mistress, in 1121. He had three children with her: