 In a letter to Cynethryth and Offa's son Ecgfrith, Alcuin advises him to follow the example of his parents, including his mother's piety.Elsewhere Alcuin refers to Cynethryth as "controller of the Royal household".The date of Offa and Cynethryth's marriage is not known, but it was not until after the birth of Ecgfrith that Cynethryth began to witness charters.