String Quintets op. 18 and 87-The genesis of Mendelssohn?s String quintets is closely connected to his friend Eduard Rietz and the latter?s brother Julius. Deeply affected by Eduard?s death in 1832 Mendelssohn exchanged the ?Minuetto?movement in his A major Quintet Op. 18 written six years previously for a newly composed adagio. It was in this form that the work was finally sent to the publisher. The Quintet in B flat major was onlypublished after Mendelssohn?s death; numerous unauthorized entries by Julius Rietz thus found their way into performance practice. An examination of the autograph allowed us to return the work to its original form thecommentary contains detailed information on this.