Mozart arranged Four Preludes and Fugues of J. S. Bach in Vienna in 1782. The Adagios are original compositions by Mozart, and the Fugues are his arrangements of three fugues from Bach’s Well-Tempered Clavier. The final fugue was written by Bach’s son, Wilhelm Friedemann Bach.
The arrangements were written for the Sunday musicales at the home of the Baron van Swieten, where, according to a letter written by Mozart to his father, only the works of Bach and Handel were performed, and where Mozart used to go and play every Sunday.
-- Kendall Briggs