From the Publisher
Bach had included these four pieces in Part III of the Clavierübung (1739), as this print chiefly contained works for organ (21 chorale preludes on Catechism melodies and other hymns); moreover, a spiritual-religious dimension has often been sought for the “Four Duets”. Indeed, they already evoke the raising of Bach’s late works, such as “Das Musikalische Opfer” and “The Art of Fugue”, to a spiritual level.
Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).