This massive work, requiring a good 15 minutes in performance, is one of Bach's most important and flamboyant organ compositions. Unlike most of his other preludes and fugues, this one seems to be a mature work written after Bach settled in Leipzig in 1723. Unexpectedly, it's the prelude that displays the most severe musical architecture, while the fugue is comparatively freewheeling (and harder to play). The grand prelude is cast in a verse and refrain structure, but employs three related thematic elements -- two for the verses. The refrain initially arches across 18 bars of the score; after the first verse, it returns in the dominant, establishing the tonality for the second verse. Here, Bach introduces the third theme, a dotted figure linked to the themes of both the refrain and the first verse. After this second verse, the refrain returns, modulating into the subdominant, where the third verse develops the themes presented in the first two. The refrain returns one last time; under the influence of the dominant pedal note that introduces it, the refrain avoids returning to the tonic until its very last chord. Despite its strict melodic structure, the prelude is a great harmonic adventure.
Now comes the fugue, which manages to fall into ternary form while following the usual fugal conventions. The first of the three sections is a self-contained fugue, complete with its own exposition, modulations, and episodes. The fugue theme is something of a chromatic wedge expanding around a tonic point, this wedge giving the work its nickname. The theme picks up two chromatic countersubjects during the first exposition. After the harmonic tension and surprise of this first section, the fugue's second section settles into the principal key. This portion is a 100-measure toccata, full of extremely virtuosic runs. The fugue theme pops up now and then and is also echoed in the pedal material, but doesn't fully reassert itself until the third panel of this triptych, which until near the end is a note-for-note repetition of the first section. The work ends, however, in a resplendent Picardy third, concluding this otherwise minor-mode fugue in a blaze of E major. ~ All Music Guide
