Evidence for assumptions about the transmission of written polyphony in the fifteenth century comes primarily from two sources: archival references, both letters and pay records; and music manuscripts, the collections of the polyphony performed at a given centre. Unfortunately it is all too rare that specific archival references can be associated with surviving manuscripts of fifteenth-century polyphony. For some centres manuscripts have survived but not the archives, while for others there are archival references to the copying of music, but the manuscripts have disappeared. Nevertheless, there is ample evidence to indicate that many large manuscripts were compiled from what Charles Hamm termed ‘fascicle-manuscripts’ – small gatherings of bifolios just large enough to contain one long work or several short ones. By means of fascicle-manuscripts music could circulate randomly, travelling with singers from court to court, or it could be transmitted intentionally, as when the choir at one centre commissioned a piece from a composer working elsewhere. According to this argument, the large manuscripts we are familiar with may not even have been the normal repositories for music in this period. They were created only when enough fascicle-manuscripts had accumulated. These would then be recopied into a single manuscript, providing a greater degree of permanence. The manuscript San Pietrob80, a large choirbook which can now be shown to have been copied for the choir at the basilica of San Pietro in Vaticano, represents one of those rare instances when archival data can be matched to an existing manuscript.