It is commonly believed that these letters written on the bass staff,
thus:
[Music]
represent the exact pitch, as near as may be, of this tetrachord. In
early times the lyre was tuned to these four sounds, and was called
the _Tetrachordon_; that is, four strings. This gracefully shaped
instrument has remained to this day the symbol of music. This limited
scale was extended by adding another tetrachord, which began with the
last note of the first tetrachord, thus:
A—B♭ C D
E—F G A
making a scale of seven sounds, called the scale of _Conjunct_ or
_Joined Tetrachords_; also from its seven strings, the Heptachord
scale. The next step was to take in the limit of the octave. The first
way adopted was to raise the highest string a whole tone, thus making
it the octave of the lowest; the sixth string was also raised a whole
tone to make it a whole tone below the seventh. The result was a scale
of seven sounds with one degree omitted, thus:
A—B♭ (C) D E
E—F G A
The next form was: