The subtitle of the work, commonly referred to as “The Wedge”, refers to the first half of the fugue subject, which opens up as a sort of widening, chromatic wedge around the tonic point. The tradition of descending chromatic fourths in Bach's E minor fugue subjects include the BWV 914 harpsichord Toccata, the BWV 855 Prelude and Fugue from the Well Tempered Clavier, Book 1, as well as the “Un poco Allegro” movement from the BWV 528 Organ Sonata. The fugue, clocking at 231 measures, is among Bach's longest and most elaborate organ fugues. The movement is unique in that it is in a three-part structure, with the third da capo section being a note-for-note reprise of the first. The second section suddenly thrusts the piece into an over-one-hundred measure episode of rapid, toccata-like passages of great virtuosity, with the cascading passagework occasionally giving way to the subject.
Albert Schweitzer described both movements as being “so mighty in design, and have so much harshness blended with their power, that the hearer can only grasp them after several hearings.” Philipp Spitta referred to the work as a “two-movement symphony”, commenting on the work's “life energy” and the “extreme daring” nature of the fugue subject. Peter Williams attributed the work's “riveting power” to the “easily felt balance between the two movements.”
