Among the many qualities that set Bach apart from his contemporaries is his manner of transforming his thematic material—most notably his fugue subjects. Bach himself believed that all good fugue writers should know how and when to change (or mutate) their subjects. The consequences of such changes are most often purely tonal in nature, giving rise to modulations that would otherwise take place during episodes. Much less frequently, tonal and rhythmic transformations of the subject may occur simultaneously, as in the fugue from the Sinfonia that opens Partita no. 2 in C minor for keyboard. It turns out that the subject of this fugue is so marked in rhythm and contour that it seems to motivate—and even demand—its own transformation as the fugue unfolds. One consequence of such transformation is a series of metric shifts from triple hypermeter during expositions to duple hypermeter during episodes.
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