The tonary letter system is thought to have originated in tenth-century St. Gall, possibly with Hartker. They appear in early manuscripts with adiastematic notation from this area and are written in the margins of manuscripts. Tonary letters reference a tonary (whether included in the manuscript or elsewhere), by means of a two letter system, the first of which indicates mode, and the second of which distinguishes between differentiae within a mode. Modally-indicative letters, in ascending order from mode 1 to mode 8 are: a, e, i, o, b, H, y, w. For example, 'ab' is a mode 1 differentia and 'wd' is a mode 8 differentia. Johannes Cotto, active around St. Gall c. 1100, described these letter in his De Musica: "One should also know that by some the phtongi - that is, the tones - are designated by vowels, and the differentiae of the tones...by consonants, in this way: a denotes the first tone, e the second, i the third, o the fourth, u [v] the fifth, Greek H the sixth, y the seventh, and ω the eighth. And b indicates the first differentia of any tone, c the second, d the third, g the fourth, and so on, with the mute consonants in alphabetical order" (Chapter 11). See the following example from CH-SGs 388 (p. 138):
A couple manuscripts, A-Wn 1890 and D-KA Aug. LX use tonary letters in conjunction with notated differentiae in the manuscript. For example, see A-Wn 1890, f. 165 (below), where the tonary letter "i" (indicating a third mode differentia) is written underneath the notated differentia in the margins of the manuscript. In D-KA Aug. LX, the notated differentiae seem to have been added at a later date in-line, at the end of each antiphon, and the tonary letters are written in the margins of the manuscript, many of which are trimmed off.