or "Mists" or "Fog" is the first prelude of Claude Debussy
's second set of preludes
. It can be considered as the most harmonically complex of the entire series of preludes, hinting at polytonality
. The left hand mainly employs the C diatonic collection, modulating shortly in the second theme and reverting in the coda, while the right hand uses the A-flat minor diatonic collection on E-flat, like the left hand modulating briefly before returning.