Differentiae are indicated throughout the manuscript by tonary letters in the margins that refer to a tonary (pp. 2-5), which provides the adiastematic neumes for each of the tonary letters. A second tonary, with slightly different differentiae that reference the same tonary letters, was added in the fourteenth century (pp. 495-497). This version is also indexed in the differentiae Database; see http://differentiaedatabase.com/manuscript/sankt-gallen-stiftsbibliothe… 388 (14th).
Two differentiae are notated on staff lines in the margins of the manuscript in a later hand for the following antiphons: Laetare Gallia quae tantum virum (p. 480) and Hic est ille Leonardus qui (p. 480). The differentia for Laetare Gallia (mode 4, differentia 41a), does not match the mode 4 antiphons included in either tonary, as transcribed by P. Ephrem Omlin (it rises to a C rather than a B). The differentia for Hic est ille, a mode 5 antiphon, matches the differentia "v" in the manuscript's tonary (differentia 129a). See http://differentiaedatabase.com/manuscript/sankt-gallen-stiftsbibliothe… 388 (14th).
For the rationale of the transcription of adiastematic neumed differentiae into pitched notation, see Rebecca Shaw's thesis. The differentiae in this tonary are also transcribed into pitched notation in P. Ephrem Omlin's Die Sankt-Gallischen Tonarbuchstaben (Regensburg: Friedrich Pustet, 1934).
Hammond, Susanne. Inventory of Sankt Gallen, Stiftsbibliothek, 388. Edited by Andrew Mitchell. In: Cantus: A Database for Latin Ecclesiastical Chant: Inventories of Chant Sources. Directed by Debra Lacoste (2011-), Terence Bailey (1997-2010), and Ruth Steiner (1987-1996). Web developer, Jan Kolácek (2011-).
Omlin, P. Ephrem. Die Sankt-Gallischen Tonarbuchstaben. Regensburg: Friedrich Pustet, 1934.
Images: St. Gallen, Stiftsbibliothek, Cod. Sang. 388, pp. 2-5 (www.e-codices.ch)
Standardized manuscript tonary completed by Rebecca Shaw (Dalhousie University) on May 6, 2018. Edited by Jennifer Bain, Sister Ilaria Culshaw, Lucia Denk, Andrea Klassen, and Brayden Olson (Dalhousie University) on August 29, 2019.