Prelude in C Major from WTC Book I

This most famous of piano pieces by J. S. Bach is more subtle and complex than it is given credit for. It contains harmonies from seven scales, in this order: Major, Melodic, Harmonic Major, Harmonic minor, Harmonic minor b5, Melodic b5 and Neapolitan minor.

After the first change up a fifth to G Major in bar 6, the sequence of modulations is as follows: Bars 12 - 16: G Melodic - D Harmonic Major - D Harmonic minor - D Harmonic minor b5 - D Melodic b5 - C Harmonic Major - C Major.

The next change is in the opposite direction, back a fifth to F Major and then this sequence occurs: G Melodic - G Harmonic minor - G Neapolitan minor - Eb Major - C Harmonic minor.

The final sequence is simply a parallel Major modulation, C minor to C Major: Bars 25 - 30: C Harmonic minor - C Harmonic Major - C Harmonic minor - C Melodic - G Harmonic Major - G Major - C Major.

So the prelude has three sequences, forward a fifth, back a fifth and a parallel change. This is what gives the piece its lovely balanced feel.