Bach’s Clavier-Übung III is a collection of organ pieces that follow the themes of the Lutheran Sunday services. Each piece has a clear theme. But after all the chorale arrangements, the first of four enigmatic duets… suddenly makes its appearance. Musicologists still disagree today about whether these duets have a special significance and, if so, what it might be.
The interpretations are very diverse. Some think that the four duets right before the closing fugue simply bring the total number of pieces in the collection to 27. This does at least conform to the clear trinity theme of the book: 27 = 3 × 3 × 3. But there are also theories about the cross symbolism, the traditional Easter hymn Victime paschali laudes, Lutheran prayers and devotions, and communion practices, etc. In short, the only clear thing nowadays is that if the duets did have symbolic meaning, we have still not been able to shed light on it.