His set of 24 Preludes is based on Bach’s idea represented in 24 Preludes and Fugues (one in each key signature).  The only difference is that Bach’s Preludes and Fugues are written in the chromatic order of the keys (C major, c minor, C sharp major, c sharp minor etc) and Chopin’s set follows major and its relative minor order (C major, a minor, G major, e minor etc).