The date of this piece can't be pinpointed beyond asserting that it comes from Bach's Weimar years, 1708 - 1717. The ten-bar fantasia is more complicated than it looks. On paper, it's just a series of chords taking less than a minute to play; in reality, the harpsichordist is expected to arpeggiate and improvise on the chords as lavishly as desired, exploring the chords' dissonances and harmonic surprises. The driven fugue, Bach's longest outside The Art of the Fugue, shares its basic theme with Bach's Fugue for organ in A minor (BWV 543). The restless, note-splattered music steadily percolates, thickening its texture with counterpoint derived from the main theme, modulating through several keys, and never dissipating its energy until the cadence of the final measures.

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