Intervals reference
Interval Quality
Interval quality distinguishes different sizes that share the same interval number. It explains, for example, why two kinds of third can have different sounds and theoretical roles.
Developing reference
Overview
Unisons, fourths, fifths, and octaves belong to the perfect-quality family. Seconds, thirds, sixths, and sevenths use major and minor as their basic qualities.
Number and Quality
The written letter distance determines the number first. The exact pitch distance then determines quality, which is why enharmonic spellings can produce different interval names for the same number of semitone steps.
Augmented and Diminished Qualities
Widening a perfect or major interval by one semitone produces an augmented interval. Narrowing a perfect or minor interval by one semitone produces a diminished interval; further alterations are possible but less common in introductory work.
Applications
Quality names make scale and chord formulas precise, clarify voice leading, and support accurate ear training. They also prevent a physical fret distance from being mistaken for a complete theoretical spelling.