According to our text, the chaconne was composed using a variation procedure based on a harmonic progression rather than a single melody. In this piece, the basic harmonic progression is:
: i ii(halfdim.) V i VI iv i V i :
*Since it's a solo line, the inversions of these chords change often in the piece. However, this is the basic progression of the theme, which is eight measures long, consisting of two parallel, symmetrical phrases that use the same chord progression. Since the first phrase has an authentic I don't know if Bach did this often, but the theme is displaced on the bar - it begins on beat 2 and ends on beat 1.
The piece contains more variations than I want to count. The first 19 basically move from a more chordal structure to a melodic line. At measure 81, the variations are put on hold and a turbulent period begins with a series of descending fully-diminished seven chords starts. The basic progression returns at measure 89 and the process continues, moving once more from a chordal structure to a single moving line and then back to chords.
At measure 133, the piece modulates to D Major (the parallel major), and the progression, it seems, is put on hold. This section, which lasts until m. 177, mainly consists of V - I movement. In measure 177, a resemblance of the original progression returns, this time in major. It then builds to a sort of ending for the D Major section in m. 209, and then there's another phrase modulation back to d minor, as well as the basic harmonic progression, though chords are substituted (that still maintain their function in the progression). For example, the ii half diminished chord is replaced by the iv7 chord.
There is a huge sense of return at measure 249 after a long buildup. The main progression returns and is played basically in the same way it was in the beginning.
Nice work, Bachy boy.