Well, in 1747 Johann Sebastian Bach, then 62 and not looking for a more interesting job than the one he'd held for almost 25 years at the Thomaskirche in Leipzig, heard from an old acquaintance, the polymath Lorenz Christoph Mizler von Kolof, the first person after the Renaissance to lecture on music history at a German university, and at this point in his life court physician to King August III of Poland, and the founder and permanent secretary of the Corresponding Society for the Musical Sciences (Correspondierende Societät der musikalischen Wissenschaften); Bach was invited to become the society's 14th member. In return, he presented the society with a work of fantastical complexity and weirdness, Some Canonic Variations on the Christmas Song "Vom Himmel Hoch", and a portrait of himself, the famed painting by E.G. Haussmann, in which he is shown holding a copy of another one of those crazy last works, the triplex canon for six voices BWV 1076.
The melody had been composed by Martin Luther a couple of centuries earlier, and Bach apparently liked it a lot, having set it several times in church music works. A canon is a polyphonic work in which different voices play or sing the same melody starting at different times (Row, Row, Row Your Boat is a canon) and often at different pitches and sometimes different tempos. These five start off as two-part canons in the organist's left and right hands while the feet play the original Luther melody on the pedals, but they get more complicated. The graphics in this video do a brilliant job of showing how it works:
