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Consonant intervals (unison, octave, perfect fourth, perfect fifth, major/minor third, major/minor sixth) sound stable and require no resolution. Dissonant intervals (major/minor second, major/minor seventh, tritone) sound tense and typically resolve to consonant intervals. This distinction between tension and resolution is fundamental to harmonic progression and voice leading.
From your work on interval quality and semitone counts, you can already identify any interval by its size and quality — you know that a major seventh spans eleven semitones and a perfect fifth spans seven. Consonance and dissonance take that knowledge one step further: they tell you what those intervals *do* to a listener and how they behave in musical time.
Consonance describes intervals that sound stable, complete, and self-sufficient. When you play a perfect fifth (C and G, seven semitones), nothing about the sound demands continuation — it could end there. The same is true of thirds and sixths, which give tonal music its warm, "in tune" quality. Dissonance describes intervals that sound tense, incomplete, and unstable. Play a major seventh (C and B, eleven semitones) and you immediately feel a pull toward resolution — the B wants to move up to C, or C wants to move down to B, collapsing the gap into a unison or octave. The most extreme dissonance in Western tonal music is the tritone (augmented fourth / diminished fifth, six semitones, like C and F#): it splits the octave exactly in half and generates maximum instability. Medieval theorists called it *diabolus in musica* — the devil in music — because its restless quality is so difficult to ignore.
The physics of consonance involves the overtone series: when a string vibrates at a fundamental frequency, it also vibrates at integer multiples — the 2nd harmonic (octave), 3rd harmonic (fifth), 4th, 5th (major third), and so on. Intervals that appear early in the overtone series are consonant because the two pitches share more overtones and "fit" together acoustically. Intervals appearing later in the series — or not at all — generate more acoustic friction. This is why perfect intervals feel purest and thirds feel sweeter than seconds.
What makes this genuinely musical — not just acoustic — is that dissonance has function: it creates the expectation of resolution, and resolved dissonance is the engine of tonal harmony. The classic example is the dominant seventh chord (C–E–G–B♭ in G major): it contains both a tritone (between B and F) and a minor seventh, stacking two dissonances that urgently demand resolution to the tonic chord. The tritone wants to resolve inward to the tonic's third and fifth; the seventh wants to step down. When the chord resolves, the tension releases. This cycle of tension and resolution — dissonance sought and dissolved — is what gives tonal music its sense of motion, arrival, and meaning. Understanding consonance and dissonance is not just labeling intervals; it's understanding why music feels like it goes somewhere.
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