Brouillards or "Mists" or "Fog" is the first prelude of Claude Debussy

Claude Debussy

Claude-Achille Debussy was a French composer. Along with Maurice Ravel, he was one of the most prominent figures working within the field of impressionist music, though he himself intensely disliked the term when applied to his compositions...

's second set of preludes. It can be considered as the most harmonically complex of the entire series of preludes, hinting at polytonality

Polytonality

The musical use of more than one key simultaneously is polytonality . Bitonality is the use of only two different keys at the same time...

. 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 G-flat minor diatonic collection on E-flat, like the left hand modulating briefly before returning.