Bach. Prelude & Fugue No. 2. The first minor Prelude & Fugue from his Well-Tempered Clavier - written to show the possibilities of equal temperament (where you can play in tune no matter what key you play). Before this, music was often centered around each key, where if you were in tune in C major, you were out of tune in F# major (or vice versa) - this was a problem if you wanted to change keys. So to show how equal temperament solved this problem, Bach wrote two books of Preludes & Fugues, writing one for each of the 12 major and minor keys - probably not realizing that he had just written one of the most important musical works of all time. I hope you have a great Monday ♥