In musical theory, a tetrachord is a series of four diatonic tones encompassing the interval of a perfect fourth. Two similar tetrachords, a tone apart, form the diatonic major scale. Originally tetrachord referred to the four strings (tetra - chord) of Greek lyres of which two, the outside or lowest and highest, were fixed (in a just, 3:4, perfect fourth) and two, the inside or middle strings, were movable. Arab music divides the tetrachord differently than the Greek.
For example, al-Farabi presented ten possible intervals used to divide the tetrachord (Touma 1996, p.19):
A diatonictetrachord has a characteristic interval that is equal to, or less than half the total interval of the tetrachord (or 249 centss).
A chromatictetrachord has a characteristic interval that is greater than half the total interval of the tetrachord, yet not as great as four-fifths of the interval (between 249 and 398 cents).
Since there are two tetrachords and a major tone in an octave, this creates a 25 tone scale used in the Arab tone system before the quarter tone scale.
The main characteristics of this tetrachord are therefore a "small" semitone (s) of 90 cents (the equal temperament semitone is by definition 100 cents) and two "large" tones of 204 cents (T).
Since the four degrees of the diatono tetrachord are taken, by definition, from the same series of pure fifths, the two intermediate degrees of the sintono must form part of a series which is higher by a comma.
the tetrachords, are not affected in the sense that their ratios one to another are maintained, both for the extreme strings and stepwise from degree to degree.
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