In music theory, harmonic rhythm, also known as harmonic tempo, is the rate at which the chords change (or progress) in a musical composition, in relation to the rate of notes.
[2] Thus a passage in common time with a stream of sixteenth notes and chord changes every measure has a slow harmonic rhythm and a fast surface or "musical" rhythm (16 notes per chord change), while a piece with a trickle of half notes and chord changes twice a measure has a fast harmonic rhythm and a slow surface rhythm (1 note per chord change).
According to Walter Piston (1944), "the rhythmic life contributed to music by means of the underlying changes of harmony.
The pattern of the harmonic rhythm of a given piece of music, derived by noting the root changes as they occur, reveals important and distinctive features affecting the style and texture.
For example, the first prelude (BWV 846) from J. S. Bach's The Well-Tempered Clavier, illustrates a steady harmonic rhythm of one chord change per measure, although the melodic rhythm is much faster.