Interval quality
The quality of an interval is its relative size compared to similar intervals. Commonly used terms for qualities include major, minor, perfect, augmented, and diminished.
The relative quality of an interval is defined in terms of the ambient scale where it occurs, while its absolute quality is defined based on a fixed scale independent of the context where it occurs, usually the diatonic scale.
Quality can also be expanded to chords. Chord qualities are related to the qualities of the component intervals that define the chord.
Relative interval quality
In any scale, each interval class consists of the set of all intervals that span a given number of steps. For example, all intervals that span two steps of a scale are thirds or 2-steps (the latter form being often used to avoid confusion with absolute interval quality and to make interval arithmetic more intuitive for unfamiliar scales). Scales with a higher density of notes typically have smaller 2-steps; as a result, in a scale with more or fewer notes per octave than the diatonic scale, the 2-steps may fall outside of the usual range for diatonic thirds (i.e. between 240 ¢ and 480 ¢).
In an equal scale, each interval class contains a single perfect interval; in other words, each interval is perfect. Therefore, both intervals 5\8 and 5\13 are perfect 5-steps (or perfect sixths) within their respective edo taken as a scale, even though they have significantly different sizes.
In moment of symmetry (MOS) scales, each interval class contains two intervals except for the unison class, which only contains the unison class. The two interval classes that correspond to the |bright and dark generators contain only perfect intervals except for one, which corresponds to the "wolf" interval, which is qualified as either augmented or diminished depending on its size relative to the perfect generator, or sometimes imperfect. The other interval classes contain major and minor intervals.
Scales with higher interval variety require additional qualities. Although there are no standard labels yet, large, medium and small can be used for variety-3 interval classes.
The harmonic series taken a scale theoretically contains infinitely many interval qualities for each interval class. For that reason, relative quality is rarely used in that context and other tools are used to describe the variety of intervals found in just intonation taken as a whole, such as absolute interval quality.
Absolute interval quality
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Todo: expand
Expand "absolute quality" (similar to interval regions). |