Prelude and Fugue in A minor, BWV 543

Prelude and Fugue in A minor, BWV 543 is a piece of organ music written by Johann Sebastian Bach sometime around his years as court organist to the Duke of Saxe-Weimar (1708–1717).

An alternate version of the organ piece is numbered BWV 543a. As for this earlier version only the prelude is different, that version of the prelude is sometimes indicated by BWV 543/1a (BWV 543/2, that is the fugue, being identical in both versions).

The fugue of BWV 543 is the final incarnation of Bach's harpsichord Fugue in A minor, BWV 944, written in 1708.

The musical materials of the prelude are a descending chromatic bassline and simple arpeggiated chords above it; this is first stated solo in the manual, and, after a lengthy embellishment of a tonic pedal point, in the pedal. The highly embellished cadence that follows—full of manual runs over sustained pedal notes—leads into a contrapuntal exploration of the opening material in sequence; this is followed by a very free peroration. The -like prelude—in the stylus phantasticus—bears the marks of Bach's early, north German-influenced style, while the fugue could be considered a later product of Bach's maturity.

The fugue is in 6
8 time, unlike the prelude, which is in 4
4 time. The fugue theme, like that of the prelude, is composed of arpeggiated chords and downward sequences, especially in its later half. Due to the sequential nature of the subject, the majority of the fugue is composed of sequences or cadences. The Fugue ends in one of Bach's most -like, virtuosic cadenzas in the harmonic minor. Unlike most of Bach's minor-key keyboard works, it ends on a minor chord rather than a picardy third.

...
Wikipedia