Thirds and sixths are imperfect intervals because they occur in both major and minor forms, yet their simple ratios make them pleasant sounding

Intervals classified as perfect (octave, fifth, fourth) have only one pure form. But thirds and sixths occur in two forms — major and minor — which is why they are called imperfect intervals. Despite the name, they are not inferior: their frequency ratios are comparatively simple (major third = 5:4; minor third = 6:5; major sixth = 5:3), giving them a very agreeable sound. Because of their harmonious quality, thirds and sixths are widely used in harmony — backing vocalists naturally harmonize by singing a third or sixth above a melody. Seconds and sevenths have more complex ratios, creating rougher (discordant) sounds.

Examples

A major third (C-E, 4 semitones) and a minor third (C-Eb, 3 semitones) are both imperfect thirds. Harmony vocals in pop songs typically follow the lead by a third or sixth.

Assessment

Explain why thirds are called imperfect. Give the semitone count for: major third, minor third, major sixth, minor sixth. Why do seconds sound harsher than thirds?