A chaconne (ciaccona in the Italian form, which Bach used) is basically a set of continuous variations over a repeating harmonic pattern (and/or its bass line). This protean one moves in the rhythm of a sarabande (in 3/4, with the weight on the dotted second beat). It is in three-part form, with an exalted middle section in the parallel major.
“On one stave, for a small instrument, the man writes a whole world of the deepest thoughts and most powerful feelings,” Brahms wrote to Clara Schumann about the Chaconne. “If I imagined that I could have created, even conceived the piece, I am quite certain that the excess of excitement and earth-shattering experience would have driven me out of my mind.”
The Chaconne has inspired reworking by later musicians in a multitude of transcriptions and arrangements, and has prompted extravagant ideas about the inner nature of its mysteries. The German musicologist Helga Thoene has developed a theory that the Chaconne is in fact a tombeau, a memorial piece for Bach’s first wife, Maria Barbara, who died in 1720 unexpectedly while Bach was away with his employer, Prince Leopold of Cöthen. In her theory, which has been realized in several recorded arrangements, the Chaconne includes many references to pertinent chorales.
— J. H.