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The seventh scale degree (the leading tone) has a strong gravitational pull toward the tonic due to its semitone distance. By ear, you hear this tension and anticipate its resolution to the tonic, understanding one of the most fundamental relationships in tonal music.
From your work singing scale degrees by ear, you know that each pitch in a major scale has a specific relationship to the tonic — scale degree 5 (sol) feels stable but active, scale degree 4 (fa) has a slight downward pull, and scale degree 2 (re) wants to resolve somewhere. But no scale degree has a stronger directional pull than scale degree 7, the leading tone (ti in solfège). Its name says exactly what it does: it *leads* — specifically, it leads upward by semitone to the tonic.
The reason for this pull is acoustic and perceptual. When you're in C major and you sing B (scale degree 7), that pitch is just one half step below C. Your auditory system has learned, through immersion in tonal music, that this semitone distance is inherently unstable — the B wants to resolve upward to C. If you sing up the C major scale and stop on B, you'll feel the pull immediately. It's almost uncomfortable to leave the phrase there. That discomfort is the leading tone doing its job.
The leading tone's power comes from its role in the dominant chord. The V chord (G–B–D in C major) contains scale degree 7 as its third. When V resolves to I, the leading tone B moves up to C — that single half-step resolution is one of the primary mechanisms by which the dominant-to-tonic relationship feels so conclusive. In the V7 chord (G–B–D–F), the leading tone's upward pull combines with the seventh's downward pull to create a two-voice resolution that defines tonal motion in common-practice music. You are already hearing this every time you recognize a V–I cadence — the leading tone is the active ingredient.
An important distinction arises in minor keys. In natural minor (using only the key signature), scale degree 7 is a whole step below the tonic, not a semitone. This pitch is called the subtonic (b7), and it does not have the same leading-tone pull — it sounds modal or unresolved when treated as a scale-ending pitch. Composers who want the strong leading-tone resolution in minor raise scale degree 7 by a half step (creating the harmonic minor scale). This is why you see raised 7ths in the dominant chord of minor-key pieces: the composer is deliberately creating a leading tone where the key signature doesn't provide one. By ear, the difference is audible — the raised 7th (true leading tone) creates tension and forward pull; the subtonic (unraised 7) sounds settled or archaic, with no strong gravitational tug toward the tonic.
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