Tritone Resolution Direction and Voice-Leading

Core Idea

Tritones (augmented fourths and diminished fifths) must resolve by contrary motion: augmented fourths expand outward to a fifth; diminished fifths contract inward to a fourth. This resolution convention drives harmonic closure and voice-leading direction.

Explainer

The tritone — an interval spanning exactly three whole steps, equivalent to six half steps — has been called the *diabolus in musica* by medieval theorists. Its sonic character is unstable and demanding: neither the brightness of a perfect fifth nor the sweetness of a third, the tritone creates a tension that strongly implies motion. Understanding *which direction* that motion goes is one of the most practically useful concepts in voice leading, because the tritone appears in every dominant seventh chord and therefore governs every authentic cadence.

From your prerequisite work with interval quality, you know that the tritone appears in two enharmonically equivalent forms: the augmented fourth (like F to B♭ in C major — wait, actually F to B in C major) and the diminished fifth (like B to F in C major). These are the same size in equal temperament but resolve differently because of the voice-leading context. The rule is: augmented fourths expand outward to a major sixth or fifth, while diminished fifths contract inward to a major third or fourth. The direction of resolution is determined by which note is the leading tone and which is the fourth scale degree — the same two notes, but the direction each moves depends on which one is "above" and which is "below."

The most important application is the dominant seventh chord. In C major, the G7 chord contains the tritone B–F: B is the leading tone (scale degree 7, pulling upward to the tonic C), and F is the fourth scale degree (pulling downward to the third, E). When this chord resolves to I, B moves up a half step to C and F moves down a half step to E — contrary motion, each moving toward their nearest resolution. This is not a stylistic convention but a consequence of the natural tendency of each pitch within the key. The leading tone *must* ascend; the fourth degree has strong gravitational pull downward. The tritone resolution is the acoustical and tonal logic that makes the V–I cadence feel inevitable rather than arbitrary.

This principle extends to every chord containing a tritone, including diminished seventh chords (which contain two tritones simultaneously, creating even greater tension) and the leading-tone triad (vii°). In each case, identifying the tritone within the chord and applying the expansion/contraction rule predicts how the chord should resolve. When you violate these resolutions — holding a leading tone down, or moving the fourth scale degree upward — you create a friction against expectation that, in tonal music, reads as an error or an intentional deceptive effect. Fluency with tritone resolution is therefore not just a rule to follow but a tool for understanding *why* tonal harmony sounds the way it does: the gravitational pull of half-step resolutions, directed by contrary motion, is the physical mechanism underneath harmonic tension and release.