Consonance and dissonance · Inversion and consonance · Music theory
A consonant interval sounds stable, at rest; a dissonant one sounds tense and asks to move on. Traditional theory, the one asked about in exams, classifies…
Consonance and dissonance
A consonant interval sounds stable, at rest; a dissonant one sounds tense and asks to move on. Traditional theory, the one asked about in exams, classifies them like this.
The perfect consonances are the unison, the octave and the perfect 5th. The perfect 4th is also considered consonant on its own, although in harmony, above the bass, it is treated as a dissonance. The imperfect consonances are the major and minor 3rds and 6ths. The dissonances are the 2nds and 7ths and all augmented and diminished intervals.
This classification is not a law of the ear: it changed over the centuries. In the Middle Ages the 3rd was considered dissonant, and many kinds of music today treat the 7th as a stable color. It serves to understand classical harmony, where each dissonance asks for a resolution.
Where consonance comes from
When a note sounds, its harmonics actually vibrate with it: exact multiples of its frequency. The first ones form the octave, 5th, 4th and major 3rd, in that order. Those simple ratios have been known since Pythagoras, who studied them on the monochord, and in the 19th century Helmholtz proposed a physical explanation: the more the harmonics of two notes coincide, the fewer beats there are and the more stable the interval sounds.