Harmonic rhythm is the rhythm created by each chord change. The harmony might change with each bar, twice a bar, with each beat, or perhaps every two bars – there are many possibilities. Harmonic rhythm can be regular, irregular or static (unchanging).
This piece is the Trio section of Mozart’s German Dance K605 no.3.
The harmony alternates between chords I and V7, with two V7 chords in the middle two bars of the 4-bar phrase. Notice though, that Mozart does change the inversion of the chord, when it is repeated in bar 3. The change of bass note helps keep the momentum going, and also makes the piece a little more interesting.
The harmonic rhythm of this excerpt could be shown like this:
This harmonic rhythm is regular, which means each chord changes at the same speed.
In this example, The Miller of Dee (folk song), the harmonic rhythm does not always move at the same speed – it is irregular.
We can show the harmonic rhythm like this: