Harmonic Sequence

A harmonic sequence is a pattern of usually two chords that gets transposed by the same interval over and over. Because the interval of transposition never changes, your ear locks onto the pattern after the first repetition and predicts where it is headed. That is why a descending chain of chords feels like it is walking downstairs on its own: the engine is not any single chord-to-chord pull but a mechanical shift, the same distance, in the same direction, again and again.

The most common version is the descending fifths sequence, where each root drops a fifth, or equivalently rises a fourth, and the chord quality is whatever the key signature gives that scale degree. In C major starting on I, that gives I, IV, vii-dim, iii, vi, ii, V, I, each root a fifth below the last. The descending 5-6 sequence works differently: a root-position chord moves to a first-inversion chord a third below it, alternating the intervals 5-3 and 6-3 above the bass so the upper voices keep shifting between a fifth and a sixth above the bass while the bass itself descends stepwise.

A melodic sequence often rides on top of a harmonic sequence, and the two move in lockstep: if the harmony transposes down a fifth each time, the melody above it transposes down a fifth too, repetition for repetition. That shared interval is what ties this topic to nonchord tones and motivic work elsewhere in the unit, since the repeating melodic cell riding the sequence is frequently decorated with the same passing tones or suspensions each time it reappears.