Western plainsong studies have typically focused on fully notated manuscripts, which provide the most complete witnesses to the repertories that have interested scholars in the field. Recent work, however, has shown that partially notated manuscripts, fragments, and marginalia can yield different kinds of insights into manuscript culture, as well as the uses and functions of musical notation. This article explores how a partially notated manuscript preserving the Old Hispanic rite, Toledo, Cathedral Archive, MS 35–6 (T6), can expand our knowledge of Old Hispanic chant, its scribal practices, manuscript culture, and notation. We identify the specific palaeographical traits and melodic dialects associated with each scribe. On this basis, we hypothesize that scribes used notation for a variety of reasons: to train in singing and writing, to practise writing, to correct particular melodies and notational forms, to preserve particular versions within a variant melodic tradition, and as an aide-memoire. T6 offers new insights into the various ways that the Old Hispanic oral tradition could be supported by writing.