Consonance and dissonance: why some chords feel tense

Physically, from how the overtones of two notes line up. An octave doubles the frequency exactly, a perfect fifth is a ratio of three to two, a major third four to five — simple ratios, overlapping overtones, and a sound the ear reads as fused.

A minor second sits at a ratio near sixteen to fifteen. The overtones almost coincide without quite doing so, and the beating between them is heard as roughness.

But physics is only half of it. Which intervals count as consonant has changed over centuries: the third was treated as a dissonance in medieval theory and is a pillar of harmony now. Custom does as much work as acoustics.

Perfect consonance: unison, octave, perfect fifth. Stable to the point of emptiness — which is why parallel fifths sound hollow rather than harmonious.

Imperfect consonance: major and minor thirds and sixths. Settled but coloured, and the basis of triadic harmony.

Dissonance: seconds, sevenths, the tritone, and all diminished and augmented intervals. Each carries an expectation of where it should go — the tritone in a dominant seventh resolves outward or inward by half step, and that pull is what makes V–I work.

The perfect fourth is the awkward one: consonant between upper voices, dissonant against the bass. Centuries of counterpoint rules exist because of that single ambiguity.

Dominant seventh (G7). Contains a tritone between its third and seventh. The mildest and most useful dissonance in tonal music.

Diminished seventh (Bdim7). Two stacked tritones, no note more stable than any other — which is why it can resolve almost anywhere and why film scores lean on it.

Suspended chords (Csus4). The fourth replaces the third and pulls down to it. Tension without darkness, which is why they open so many pop songs.

Minor second and cluster voicings. Two adjacent notes sounded together. On guitar, adding an open B string to a C chord high up the neck gives the effect immediately.

Altered dominants (G7♭9, G7♯11). Jazz harmony intensifying an already unstable chord — dissonance treated as colour rather than as a problem.

Classical practice treats dissonance as something entered gently and left by step: a note held from the previous chord becomes dissonant when the harmony moves under it — a suspension — and then falls to a consonance.

Popular music mostly skips the preparation and keeps the resolution. A sus4 chord that becomes a major chord is exactly that gesture, compressed.

Jazz and much film music keep the dissonance unresolved on purpose. A piece can end on a major seventh chord and sound finished, which would have been unthinkable in 1750 — evidence that resolution is a convention rather than a physical requirement.

If a progression sounds bland, it usually lacks dissonance rather than lacking chords. Turning one triad into a seventh, or delaying a chord's third with a suspension, adds motion without adding harmony to justify.

If a passage sounds unpleasant rather than tense, check the register. The same interval is far rougher low than high — a minor second in the bass muddies where the identical interval two octaves up sparkles.

And check duration. Dissonance that passes is heard as movement; dissonance that sits becomes the sound of the piece, which may be what you want.

Consonance and dissonance are properties of intervals before they are properties of chords. Managing the tension across several lines at once is counterpoint, and resolving it at the end of a phrase is what cadences do.