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5.2.6 Church Modes Scales and the Fifth Interval
For purposes of creating harmony, five Church mode scales fell into disuse. One of the scales does not even have a perfect fifth interval, which is a necessity for a successful scale.
A “successful” scale (as far as your brain is concerned) needs a mixture of two kinds of intervals:
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Easily-processed simple-frequency-ratio intervals. These intervals provide your ear with a sense of tonal recognition, a “home,” a centre of gravity.
A note associated with the next-simplest frequency ratio after the octave, namely, ratio 3:2 (scale degree 5) must be positioned in the middle of the scale. It functions as a stable counterweight to the tonic note.
At scale degree 5, a tune has travelled as far away from “home” as it can get. Now it can only proceed either downwards towards scale degree 1, or upwards towards scale degree 1 (8).