It was Charles Hamm who first formalised the distinction between fifteenth-century manuscripts whose contents are arranged primarily according to liturgical type (introits, Kyrie settings, Gloria settings, etc.), in a more or less predetermined succession of gatherings, and those in which the repertory is more arbitrarily organised through small clusters of related works and in which gatherings tend to form separate and independently conceived entities. Trent 87 and 92 (hereafter Tr87 and Tr92) belong primarily to the second category. Haberl first distinguished them from the later Trent codices as an older group of manuscripts of independent origin; but it was Adler and Roller who recognised their important subdivisions. The main section of Tr92 (fols. 1–143) and the latter part of Tr87 (fols. 219–265), they noted, had each once formed a separate book,6 while the remaining portions of the manuscripts, TY87, fols. 1–218, and Tr92, fols. 144–268 (here referred to individually as Tr871, and Tr922 but jointly as TR?), were so closely related as to suggest that they belonged together.