Gregorian chant and certain other folk songs in Western music use scales called Gregorian modes.
What are Gregorian modes?
Gregorian modes refer to the use of four scales, the scales from the dictionary of music are re, mi, fa, and so.
If the above scales are not used under the Gregorian chant, this will be referred to as the Dorian, Phrygia, Lydian, and Mixolydian.
The Gregorian modes are numbered from 1 to 8, in which the odd number equals the original modes whereas the even numbers equal the plagal modes.
Therefore, the correct fill-in-the-blanks are Gregorian modes.
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