Research Notes:

The Chronicle of Adémar de Chabannes names "Willelmus…cognomento Caput stupe" as one of the two sons of "Eblo duce", specifying that he was "Arvernis, Vallatis, Lemovice et Pictavis comes…dux Aquitanić". The Chronico Comitum Pictavić names "Willelmum Caput-stupć" as son of "Ebles Dux Aquitanić et Pictavić Comes" & his wife Adellia. Ademar names "Willelmum Caputstupć" as son of Eble and "Adelam, filiam Rosi Rotomagensis", but evidently confuses the latter with Guillaume's own wife.

He succeeded his father as GUILLAUME I “Tęte d'Etoupes/Caput-stupć” Comte de Poitou. He was appointed lay abbot of Saint-Hilaire-de-Poitiers in Jan 942. From the start of his reign, his possession of Poitou was disputed by Hugues "le Grand" Duc des Francs [Capet]. "Guillelmus comes vel abba summi pontificis domni nostri Hylarii" donated property "in pago Pictavo in viccaria Pictavis" to the church of Saint-Hilaire de Poitiers by charter dated Jun 941 or 942. Louis IV King of France confirmed the property of Saint-Hilaire de Poitiers, in the presence of "Guillelmus comes et marchio et frater eius Ebolus atque Rotgarius comes", by charter dated 5 Jan 942.

Comte d'Auvergne et de Limoges 955. Around this same time, Lothaire King of France extended the authority of Comte Guillaume over the whole of Aquitaine. Although known to history as GUILLAUME III Duke of Aquitaine, charters record him as "Guillelmus…Aquitanici ducatus comes" and "Guillelmus…Pictavensium sive Lemovicensium necne et Arvernensium comes insuper etiam Aquitainić comes palati" as well as "Willelmi duci Aquitanorum cognomento Caput-Stupć".

He abdicated in 962, and became a monk at Saint-Cyprien de Poitiers.

The Chronicle of Adémar de Chabannes records that "Willelmo Capite stupć" was buried "apud ecclesiam Sancti Cypriani". ²