Interval quality

The interval quality of an interval within a scale describes the relative size of the interval compared to the other intervals of the same interval class.

In the diatonic scale, which is an MOS scale, there are two interval qualities for each interval class, except the unison. The unison is always qualified as perfect, the fifth can be perfect or imperfect, while other intervals are qualified as major or minor. This set of interval qualities can be generalized to all MOS scales by replacing the fifth by the interval class of the generator.