A change of chord generally occurs on an accented beat, so that chord progressions may contribute significantly to the rhythm, meter and musical form of a piece, delineating bars, phrases and sections. Stewart MacPherson, Musical Form , Chapter 1, Joseph Williams, London, 1930.
Although all this allows for a large number of possible progressions (depending upon the length of the progression), in practice progressions are often limited to a few bars' length and certain progressions are favoured above others: there is a certain amount of fashion in this and a chord progression may even define an entire genre.
Source: Wikipedia > Chord Progression
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