Ever since Hugh Baillie and Philippe Oboussier's pioneering study of York, Borthwick Institute MS Mus 1, better known as the York Masses, it has been generally accepted that its compositions, if not the choirbook itself, originated elsewhere than York. Two locations claimed primacy in their bid for the manuscript's original provenance, Lincoln and London, owing to the internal evidence of two composers named in the manuscript, 'Johannes Cuke' and 'Horwod'. The evidence is reassessed here with regard to an important new source relating to polyphonic music and other fragments of music preserved in post-Reformation York bindings. It is suggested that these fragments originated at one or more churches in York in the late fifteenth century, and that they were finally sold for binding material c. 1583, resulting in their appearance in the same series of court books for York Minster. The cultural background for the genesis and performance of polyphonic music is then addressed, with reference to York and other northern locations such as Durham, Beverley and Selby.