The source is the organ work composed by J. S. Bach, a master in music history. Bach was also an organist active as a court musician in the early 1700s. Many organ works came from this period, "Passacaglia and Fugue" being one of them.
Passacaglia is a type of variation with an ostinato (repetitive) theme in the bass part. The original work is in long form with the theme in the first 8 measures, 20 variations, and the fugue. This arrangement is an excerpt of the presentation, nine variations, and the fugue (conclusion).

(Sawako Yamazato)