A passacaglia is a Baroque dance that is a lot like the chaconne. One of Bach’s greatest hits is his Passacaglia and Fugue in C minor. Like the Chaconne, the Passacaglia is a long series of variations on a short, simple dance form. Also like the Chaconne, it’s pretty awesome.
Bach got the first half of the theme from André Raison’s Trio en Passacaile from Premier livre d’orgue. He took it a lot further out, though.
Before we go any deeper into the music, let’s talk about this instrument. Each pipe in a pipe organ plays a single note with a particular timbre. There are multiple pipes for each note, each of which produces a different blend of overtones. The knobs all around the keyboards on the organ are called stops, and they activate and deactivate different banks of pipes to produce different timbres. A big organ will have multiple keyboards, one of which is a set of foot pedals, and each keyboard controls its own array of banks of pipes. Furthermore, each keyboard can have different stop settings, effectively making each one a separate instrument. If you think about it, that makes a pipe organ the mechanical equivalent of a modular analog synthesizer.