Seal of William, Count of Luxembourg. The Latin inscription on the border of the seal reads: SIGILLVM WILHELMI COMITIS DE LVCELENBVRCH
William I, Count of Luxembourg (1081–1131) was count of Luxembourg (1096–1131), in succession to his elder brother Henry III of Luxembourg. They were both sons of Conrad and Clementia of Aquitaine. William was the first of his family to use the title count of Luxembourg in his documents.
Like his predecessors, he was embroiled in debates with the archbishop of Trier Bruno in 1122 and 1127, leading to William's excommunication.
Around 1105 he married Mathilde or Luitgarde of Northeim, daughter of Kuno, count of Beichlingen, and had :
