Canonic Variations on "Vom Himmel hoch da komm' ich her"
Bach is often said to be a “mathematical” composer.  This set of variations are a good example of a piece of music in which Bach is trying to show different techniques of composing.  A lot of skill is needed in order to compose complicated canons like this, making the music sound good at the same time.  The description of the music may sound very complicated, but one can enjoy the music without understanding all the techniques used to compose it.  However, it is always fascinating to see how Bach has put all these ideas together.