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.
Fugue
Peter Williams points out that the catchy "lengthy sequential tail" of this fugue subject easily "easily confuse the ear about the beat" and is harmonically an exact "paraphrase" of the sequence in bars 6-8 of Vivaldi's double violin concerto Op. 3 No. 8 in A minor. Bach arranged this Vivaldi concerto as his solo organ "concerto" BWV 593, probably in 1714-16. This 4-voice fugue BWV 543 has been compared to Bach's harpsichord Fugue in A minor, BWV 944, a 3-voice fugue that was probably written in 1708, and this organ fugue has even been called "the final incarnation" of BWV 944. However, the idea of any close relationship has been challenged. Williams writes that the fugue "has often been likened to the keyboard fugue BWV 944 and claimed as some kind of version of it the resemblances — contours of subject and countersubject, a perpetuum mobile element, a rather free close — are too slight" to support the comparisons. Williams also cites similarities "between the subject’s outline and that of the A minor Fugue BWV 559, or between the pedal figures in both Preludes’ closing stages in the Prelude’s opening figure, in a Corrente in Vivaldi’s Op. 2 No. 1, of 1709, and in a Fugue in E minor by Pachelbel." Aside from Williams' observations about the fugue subject, the fugues BWV 543 and 944 differ in their larger outlines: their harmonic structure and the series of expositions and episodes are not parallel.
Score
Prelude
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 Toccata-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.
Fugue
The fugue is in time, unlike the prelude, which is in 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 toccata-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.
Arrangements
Liszt's transcription
Because of the piece's overall rhapsodic nature, many organists play this piece freely, and in a variety of tempi; it can be easily transcribed to a different instrument. Liszt included it in his transcriptions of the "six great preludes and fugues" BWV 543-8 for piano.
