The Prelude of BWV 546 spans a full 144 bars. It presses forth with great urgency after a stoic hand-against-hand incipit gesture and is soon filled with running triplets, here scalar, there arpeggiated.
The opening gesture returns twice, once in the minor dominant after about 40 bars and again in the tonic C minor to usher in the final bars of the piece.
The fugue is a five-voice one (though the five voices are heard all together only in the exposition of the fugue and at the very end, where, as per tradition, the texture blossoms even further, in this case to a final eight-voice cadence) and is built from a subject that can hardly restrain its urge to leap up by the interval of a third; it is all Bach can do to ensure that a few stepwise motions find their way into this otherwise leap-infested thought.
While the interval between the composition of the Prelude and of the Fugue from J.S. Bach's Prelude and Fugue for organ in C minor, BWV 546 may not be as great as the interval between the composition of the Prelude and the Fugue from BWV 562 (the Prelude of that work having likely been written sometime during the 1710s but the Fugue possibly as late as 1745), they are nevertheless hardly temporal bedfellows. The Fugue of BWV 546 seems to be the earlier of the two halves (it is a work from Bach's Weimar days [1708 - 1717]), while the Prelude came later (from his Leipzig days.
Description by Blair Johnston
The Praeludium closes in c-major.
Probably this is because Bach wrote it as an independent piece.
When played together with the following fugue, which is also in c-minor, baroque practice is to close the praeludium in the minor key.