As Separate Compositions
As a separate piece, a "Perpetuum Mobile" can be defined as a composition where (a large part of) the piece is intended to be repeated an (often not specified) number of times, without the "motion" of the melody being halted when a repeat begins. Canons are often intended to be performed in a moto perpetuo fashion (which, in that case, can be called canon perpetuus). In some cases the repeats of a "perpetuum mobile" piece are at a different pitch (while a modulation or a chord progression occurs during the repeatable part): some of the riddle canons of Bach's Das Musikalische Opfer are examples of this particular kind of Perpetuum Mobile/Canon Perpetuus.
Perpetuum mobile as a genre of separate musical compositions, was at the height of its popularity by the end of the 19th century. Such pieces would often be performed as virtuoso encores, in some cases increasing the tempo along the repeats.
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