As a former pupil of the late Arthur Hubert Couratin, it was perhaps inevitable that sooner or later I should become fascinated by the sanctus in the eucharistic prayer. The sanctus had preoccupied Couratin's friend and mentor, Edward Ratcliff, and Couratin in turn came under the same spell. As he led pupils through the text of the Apostolic Tradition, he would raise the questions about its ending which Ratcliff had set out in his now famous article in the Journal of Ecclesiastical History – though rarely mentioning his own support of ‘the Master’ published in the same journal. I am indebted to Arthur Couratin not only for imparting his technique in assessing liturgical texts, but also for imbuing me with the ‘sanctus fascination’. However, my later graduate liturgical studies under Ronald Jasper and the late Geoffrey Cuming, and in Syriac with Sebastian Brock, have been indispensable in equipping me with an independent perspective for investigating its function, and conjecturing on its possible origin. I have found it necessary, therefore, to place this investigation in a context far broader than Couratin and Ratcliff would have thought necessary.