Three things were necessary before these Gregorian chants could develop at all:  (1) A simple, clean-cut musical scale or systematized table of musical sounds. (2) Some definite manner of symbolizing sounds, so that they could be accurately expressed in writing. (3) A cultivation of the sense of hearing, in order that mankind might learn to distinguish between sounds that are discordant and those that sound well together; in other words, harmony.

We will begin with the scale, and review what we know of the Greek modes in order to show how they were amalgamated into our present octave system of scales.

[Tetrachords         /------|-----\    /-------|--------\ ]
[                 F:  b, c d e f g a G:  b c’ d’ e’ f’ g’ a’]
[Mixolydian          \—­+-+-+-+-+-+——­/ |  |  |  |  |  | ]
[Lydian                 \-+-+-+-+-+------/  |  |  |  |  | ]
[Phrygian                 \-+-+-+-+---------/  |  |  |  | ]
[Dorian                     \-+-+-+------------/  |  |  | ]
[Hypolydian                   \-+-+---------------/  |  | ]
[Hypophrygian                   \-+------------------/  | ]
[Aeolian or Locrian or Hypodorian \---------------------/ ]
[Notes labelled from highest to lowest:  Nete, Paranete, Trite,
Nete, Paranete, Trite, Paramese, Mese, Lichanos, Parhypate,
Hypate, Lichanos, Parhypate, Hypate, [F:  a,] Proslambanomenos.]

Under Ambrose and Pope Gregory, these modes had taken a different form.  The chromatic and enharmonic styles had been abandoned in theory, the portamento which the singers introduced into their chants being the only principle retained.  The new system was as follows: 

    [F8:  g, a, b, G8:  c d e f g a b c’ d’ e’ f’ g’ a’]
    [First nine notes labelled: 
     Hypoion., Hypodor., Hypophryg., Hypolyd./Ionian,
     Hypo-mixolyd./Dorian, Hypoaeol./Phryg., Lyd., Mixolyd., Aeol.]

In order to complete the story of the evolution of scales and clefs, we must add that the Flemish monk, Hucbald (900 A.D.), divided this scale into regular tetrachords, beginning at G, with the succession, tone, semitone, tone, forming four disjunct tetrachords,

    [F:  (g, a, b-, c) (d e f g) (a b c’ d’) G:  (e’ f+’ g’ a’)]

This division remained without influence on the development of the scale.

The first change in the tetrachord system of reckoning tones and dividing the scale was made by Guido d’Arezzo (first half of eleventh century), who divided it into hexachords or groups of six notes each.  Up to that time, each note of the scale had had a letter of the alphabet for its symbol.  It was Guido who conceived the idea of using syllables for these notes.  The story of how it occurred to him is well known:  On one occasion, hearing his brethren in the monastery choir of Arezzo, in Tuscany, sing a hymn to St. John the Baptist, he noticed that the first syllable of each line came on regularly ascending notes of the scale, the first syllable coming on C, the first of the next line on D, the first of the third on E, etc., up to A on the sixth line.  As all these syllables happened to differ one from the other, and, moreover, were very easy to sing, he hit upon the idea of using them to distinguish the notes on which they fell in the hymn.