It is so measured that rising arpeggio, only to fall and rise again in quicker values, through the dominant seventh to the heartache moment of that minor ninth, a very apogee of dissonance. Then it goes higher still to the fifth, holding to that Phrygian harmony before returning to the tonic minor and a measured fall in the bass. This is a deliberate descent to the sub-mediant, and Bach’s touch of magic, the equivalence with the dominant minor ninth. But then he gives us hope: an extended and joyful play through sequences that rise and fall within each bar, to rest finally on the mediant’s echo of that opening, that measured rise and the quickening fall. We have hardly smiled with relief when Bach pulls us back into the insecurity of the dominant of the subdominant, that V of IV acting like a bridge to a long, long discourse in the dominant, a pedal E holding firmly to itself whilst rising arpeggios and falling decorations and sequences pull and pull through innocently related keys. Longer and longer play the rising passages until short motives of imitation interrupt, treble to bass, tenor to alto, until:  a first inversion arpeggio of the dominant seventh measures out the opening rhythm. This happens twice in short succession, as though holding the progress of the music to account. A questioning perhaps before a four-fold sequence asserts the dominant and a chorded caesura. There is a pregnant, though faintly resonant silence as Bach spins the dice of tonality and chooses the subdominant to bring the music towards a waiting Allemande. The music moves through a play of subdominant to dominant, minor to major, the mix of flattened fifth and flattened ninth. It is those intervals that determine Bach as the father of ambiguity in the 20C school of jazz harmony, Arpeggio then a falling scale, and repeat and repeat again, but moving ever higher by sequence. At last five chords – merely a shorthand for closure via the expectation of a right display of the performer’s improvisatory prowess. They prepare us reverently for the tonic minor before the stately Allemande leads the music into the elegant steps of its walking dance.