Consonance and Dissonance

Interval Perfect Intervals Harmonic Series Cadence Modulation Augmented and Diminished Intervals

To the ear, intervals fall into consonant and dissonant. The useful picture, though, is not two boxes but a ladder of degrees:

Tier	Intervals	Sound	Role in harmony
perfect consonance	unison, octave, fifth, fourth	fused, hollow, no tension	skeleton: it establishes the key
imperfect consonance	major/minor thirds and sixths	coloured but stable	flesh: it decides brightness and shade
dissonance	major/minor seconds and sevenths, tritone	wants to resolve, drives forward	engine: it makes music move

Notice that the top tier consists almost entirely of perfect intervals — no coincidence, because consonance follows how much the harmonic series overlaps: simpler ratios overlap more, and sound emptier. See harmonic series.

Dissonance is not "ugly"

Dissonance is where musical motion comes from. Without it, music could only stand still:

Tension and release: a dissonance appears, expectation builds, it resolves to a consonance, the tension

discharges. This is how cadences and the whole tonal system work.

Modulation: reinterpret a dissonance as belonging to a new key and the key has shifted — see

modulation.

Colour: from the late nineteenth century onward composers began using dissonance as a stable colour in

itself rather than something that must resolve.

The line is historical

The boundary is not fixed. Thirds were once classed as dissonant and later became the simplest harmonic material; in today's popular music a seventh chord is nearly consonant. The same intervals hold different positions in different contexts, so treat this ladder as a tendency, not as a statute.

Diagram: the ladder of consonance

Listen: consonance against dissonance

Hear a hollow fifth (perfect consonance), then a major seventh (dissonance), then the tritone. The difference is not loudness; it is whether you expect something to happen next.

Common misconceptions

"Consonant means pleasant, dissonant means unpleasant." This is a functional classification, not an aesthetic verdict. Dissonance is the engine; without it music cannot move.
"The boundary is a fixed rule." It moves with history. The third travelled from dissonance to the most basic harmonic material.
"A dissonance must resolve at once." Usually true in traditional writing, but in Impressionism and jazz a dissonance can linger or even serve as an endpoint.
"Consonance depends only on the interval." Register, dynamics and orchestration all matter — a perfect consonance in the low register can easily turn to mud.

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Listen in the library

Suite bergamasque: III. Clair de luneC14:36Debussy's Clair de lune — sevenths and ninths used as continuous colour, dissonance no longer hurrying to resolveDetailsSound Ye the Trumpet CallC10:35An organ hymn — in traditional writing dissonances resolve by rule, the clearest picture of tension and releaseDetailsDanse Macabre, S.555C110:20Liszt's transcription of Danse macabre — tritones crowd the texture and dissonance keeps the whole piece restlessDetails

All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).

Sources

协和分层次（完全协和 / 不完全协和 / 不协和）及与泛音重合度的对应关系，属音乐声学与和声学通则
协和判断随时代与语境变化（如三度从不协和转为协和），为音乐史通行记载

Used for fact-checking only; all prose is original.

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