Two intervals that, when combined, span an octave (e.g., E3 - G#3, a major third, plus G#3-E4, a minor sixth). When inverted, major intervals become minor (and vice versa), diminished become augmented (and vice versa), and perfect stay perfect. The generic interval numbers of inversionally related intervals sum to 9 (third and sixth, second and seventh, etc.). Two intervals that, when combined, span an octave (e.g., E3- G#3, a major third, plus G#3-E4, a minor sixth). When inverted, major intervals become minor (and vice versa), diminished become augmented (and vice versa), and perfect stay perfect. The generic interval numbers of inversionally related intervals sum to 9 (third and sixth, second and seventh, etc.).