Consonance and dissonance
Music theory · Intervals · Level 4 · 11 min
Some intervals sound stable and restful: these are consonances. Others create a tension that calls for something to follow: these are dissonances. A dissonance is not a "wrong note": it is the engine of music, the push that keeps it moving.
Compare: the fifth C-G is full and calm; the minor second C-C♯ rubs and grates.
The traditional classification
| Category | Intervals | Character |
|---|---|---|
| Perfect consonances | unison, octave, perfect fifth (perfect fourth depending on context) | open, stable, "structural" |
| Imperfect consonances | thirds and sixths (major and minor) | full, warm, expressive |
| Dissonances | seconds, sevenths, tritone, augmented and diminished intervals | tense, demand resolution |
The perfect fourth has an ambiguous status: consonant between two upper voices, it is treated as a dissonance against the bass in classical style (the famous "six-four" that must be resolved).
Why? A bit of physics
Consonant intervals correspond to simple frequency ratios: octave 2:1, fifth 3:2, fourth 4:3, major third 5:4. Their vibrations often coincide and their overtones (the secondary pitches contained in every note) overlap. Dissonances have complex ratios (16:15 for the minor second): the waves interfere and produce "beats".
Resolution
A dissonance usually resolves by step to a consonance. The tritone is the textbook case: the augmented fourth opens out to a sixth, the diminished fifth closes in to a third.
M:4/4 L:1/4 Q:1/4=66 K:C "^A4"[FB]2 "^m6"[Ec]2 | "^d5"[Bf]2 "^M3"[ce]2 | "^m7"[G,F]2 "^M3"[CE]2 | "^P4"[CF]2 "^M3"[CE]2 |]
An evolving idea
In the Middle Ages only the octave, fifth and fourth were accepted as consonances: people sang in parallel fifths (organum). Thirds only became "sweet" in the Renaissance, through English music. In the 19th century Wagner and Debussy left dissonances unresolved; in the 20th Schoenberg spoke of the "emancipation of the dissonance". And in jazz the seventh and ninth have become stable colours: nobody minds ending on a Cmaj9.
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