Harmonic Rhythm

Every phrase has two rhythms layered on top of each other: the surface rhythm of the melody's note values, and the harmonic rhythm, which is how often the chord underneath actually changes. A melody can tick along in steady eighth notes while the harmony holds one chord for a whole measure, then suddenly changes twice in the next measure. That shift in the rate of chord change is what makes a passage feel like it is speeding up or settling down, even though the notated note values never changed. The ear tracks root motion and the pull of one harmony resolving to another, and when that motion happens more often, time feels compressed.

You can picture harmonic rhythm as its own separate staff written below the music, with nothing on it but a note or a slash marking each moment the chord changes. One chord per measure becomes a single long duration per bar; two chords per measure becomes two shorter durations splitting that same bar in half. Stacking this derived rhythm against the real rhythm of the melody shows you, visually, where the harmony is moving in lockstep with the tune and where it is moving independently, which is often where a phrase gets its sense of drive or relaxation.

This matters most at cadences, the points where a phrase comes to rest. Composers often speed up harmonic rhythm right before a cadence, packing in more chord changes per beat to build momentum, then land on the final tonic chord and let it ring for a full measure or more. That sudden deceleration after the rush of change is a big part of what makes a cadence feel like an arrival rather than just another chord.