De institutione musica IV.3: Musicarum notarum per graecas et latinas litteras nuncupatio.
It now remains, since we are about to divide the string by the rule according to the aforesaid consonances, and since this division will exhibit the necessary sounds for the three kinds of melody, to set down in the meantime the musical notes, so that when we have marked the divided line with the same little signs, the name of each may be most easily recognized. For the ancient musicians, for the sake of brevity in writing, so that it might not always be necessary to put down whole names, devised certain little signs by which the names of the strings might be marked, and they divided them by kinds and modes, aiming at the same time at this brevity, so that, if ever a musician wished to write out some melody above a verse extended in rhythmic or metrical composition, he might write above it these little signs of the sounds, finding in this wonderful way that not only the words of poems, which are expressed by letters, but also the melody itself, which was marked by these little signs, should endure in memory and for posterity. But of all these modes let us for the present arrange one, the Lydian, and its little signs through the three kinds, doing the same in the remaining modes at another time. If ever I describe the arrangement of the notes by naming Greek letters, let the reader not be troubled by any novelty. For this whole description of notes is made up of Greek letters, now diminished, now even bent, in every direction. We, however, take care not to turn aside anything from the authority of antiquity. The upper and higher little signs will be those of diction, that is, of the words; the second and lower those of the stroke.
Proslambanomenos, which may be called the added, a zeta not whole and a tau lying down; hypate hypaton, which is the principal of principals, a gamma turned and a gamma upright; parhypate hypaton, that is, the next to the principal of principals, a beta not whole and a gamma supine; hypaton enarmonios, which is the enharmonic of the principals, an alpha supine and a gamma turned backward having a little rod; hypaton chromatice, which is the chromatic of the principals, an alpha supine having a line and a gamma turned having two lines; hypaton diatonos, which is the extended of the principals, a Greek phi and a digamma; hypate meson, which is the principal of the middles, a sigma and a sigma; parhypate meson, which is the next to the principal of the middles, a rho and a sigma supine; meson enarmonios, which is the enharmonic of the middles, a Greek pi and a sigma turned; meson chromatice, which is the chromatic of the middles, a Greek pi having a little rod and a sigma turned having a little rod through the middle; meson diatonos, which is the extended of the middles, a Greek mu and a Greek pi drawn down; mese, which is the middle, an iota and a lambda lying down; trite synemmenon, which is the third of the conjoined, a theta and a lambda supine; synemmenon enarmonios, which is the enharmonic of the conjoined, a Greek eta and a lambda lying down, turned, having a little rod through the middle; synemmenon chromatice, which is the chromatic of the conjoined, a Greek eta having a little rod and a lambda turned having a little rod; synemmenon diatonos, which is the extended of the conjoined, a gamma and a nu; nete synemmenon, which is the last of the conjoined, an omega square supine and a zeta; paramesos, which is the below-middle, a zeta and a Greek pi lying down; trite diezeugmenon, which is the third of the divided, an epsilon square and a Greek pi supine; diezeugmenon enarmonios, which is the enharmonic of the divided, a delta and a Greek pi lying down, turned; diezeugmenon chromatice, which is the chromatic of the divided, a delta having a little rod and a Greek pi lying down, turned, having an angular line; diezeugmenon diatonos, which is the extended of the divided, an omega square supine and a zeta; nete diezeugmenon, which is the last of the divided, a phi lying down and a nu inverted, drawn down; trite hyperboleon, which is the third of the excellent, a gamma looking downward to the right and a half-alpha at the left looking upward; hyperboleon enarmonios, which is the enharmonic of the excellent, a tau supine and a half-alpha at the right supine; hyperboleon chromatice, which is the chromatic of the excellent, a tau supine having a line and a half-alpha at the right supine having a line behind; hyperboleon diatonos, which is the extended of the excellent, a Greek mu having an acute accent and a pi drawn down having an acute accent; nete hyperboleon, an iota having an acute accent and a lambda lying down having an acute accent.