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Harmonic function (tonic, subdominant, dominant) creates both harmonic and voice-leading tension that demands resolution. Dominant function, with its tritone interval between the third and seventh, requires specific voice-leading resolution: the tritone contracts inward to a third or sixth. Voice-leading patterns reinforce harmonic function by creating and resolving tension through pitch movement and tension-bearing intervals.
Harmonic function and voice-leading are two descriptions of the same phenomenon. You already understand harmonic function conceptually — tonic is stable home, dominant is maximum tension pointing toward resolution, subdominant is departure from tonic — and you understand voice-leading principles: smooth motion, contrary motion, avoiding forbidden parallels. This topic connects those frameworks: the reason V wants to resolve to I is not an abstract harmonic rule but a physical pull built into the individual pitches of the dominant chord.
The key mechanism is the tritone. In a dominant seventh chord (V7 in C major: G–B–D–F), the interval between B (the third) and F (the seventh) is a tritone — the most dissonant interval in tonal music. B and F do not just sound unstable together; they actively pull in opposite directions. B is the leading tone, one half-step below the tonic C, and it has a strong tendency to rise by half-step to C. F is scale degree 4, and it has a tendency to resolve downward to E, the third of the tonic chord. The tritone resolves inward: B rises to C, F falls to E, and the tension collapses into a consonant third.
This resolution is not arbitrary — it is the strongest possible voice-leading motion. Both voices move by half-step, which is the smallest possible motion, and they move in contrary directions, which is the smoothest possible contrary motion. The result is that harmonic function and voice-leading reinforce each other completely at the dominant-to-tonic cadence. When you understand this, you can hear the V–I progression differently: not as two chords changing, but as two voices under tension finding their resolution.
Subdominant function (IV and ii) creates a different kind of tension — not the urgent pull of the dominant, but a sense of departure from tonic stability. The IV chord in C major (F–A–C) shares two pitches with the tonic triad but introduces F (scale degree 4), the same pitch that becomes the tense seventh of V7. When IV moves to V, F stays (or moves minimally) and the voice-leading reinforces the functional motion from departure to tension. Understanding this chain — how individual pitch tendencies accumulate across the tonic–subdominant–dominant–tonic progression — reveals why this sequence has been the bedrock of Western tonal music for centuries.
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