There appears to be no primary source which confirms that Guillaume [I] was the son of Guy de Thiern. Du Chesne suggests that he was the son of Comte Savaric, after whom he is named in the charter dated 1113 which is quoted below, but he proposes no convincing arguments. Orbendale suggests his affiliation with Guy de Thiern, whom he would have succeeded in his share of the county, highlighting especially that Guillaume would not have been named as one of “duorum comitum” in the 1113 document if Savaric had been his father, but as “filii sui”. On the other hand, the possibility that his part of the county of Chalon passed to Guillaume [I] in some other way, maybe by marrying Guy de Thiern’s daughter, cannot be totally excluded in the absence of explicit primary sources.
He succeeded [his father] as joint Comte de Chalon. A charter dated 1113 records the foundation of the abbey of La Ferté and names “duorum comitem...Savarici...et Guillelmi”. Bouchard suggests that he appears to have acquired the remaining part of the county around the time Savaric de Vergy sold Châtelet-Chalon to the duke of Burgundy after 1113, but this would appear not to be correct given the undated charter, quoted above, which specifies that Savaric sold his part of the county to the bishop of Chalon. "Willelmus comes Cabilonis et Huo filius ducis Burgundie" donated property to la Ferté by charter dated 1147. There is no indication in the document whether it refers to Guillaume [I] or Guillaume [II] Comte de Chalon. 1
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