Within each suite are various movements named for dances, and they each have very strict structures. These movements are all masterpieces in music, and they get increasingly complex. But before these dances begin, there’s always a Prelude. In the Bach suites, it’s a way to establish the key, to establish the motives, and it’s also a kind of improvisation. And this prelude, in particular, is revered because it achieves a lot with just a few very simple concepts.