orm. 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.

[F: c d f (d e) d | d d c d e e ]
[W: _Ut_ queant laxis | _Re_sonare fibris ]
[F: (e f g) e (d e) c d | f g a (g f) d d]
[W: _Mi_ra gestorum | _Fa_muli tuorum ]
[F: (g a g) e f g d | a g a f (g a) a | (g f) d c e d ]
[W: _Sol_ve polluti | _La_bii reatum | Sancte Joannes]

Furthermore, as there were six of these syllables, he arranged
the musical scale in groups of six notes instead of four,
hexachords instead of tetrachords. Commencing with G, which
was the lowest note of the system in Hucbald's time, the first
hexachord was formed of G A B C D E; the second, following the
example of the Greeks, he made to overlap