Both JS Bach's six Trio Sonatas and Mendels-sohn's six Organ Sonatas are conceived as tutorial pieces - an "organ school" - to help attain demanding playing skills at the highest musical level. We know from Bach that he wrote his sonatas for his eldest son, Wilhelm Friedemann. Mendelssohn is known to have been familiar with Bach's sonatas and their pedagogical purpose. He gradually developed his own Organ Sonatas, originally for the English market: they are equal in number to Bach's sonatas, as well as in their intention as instructive pieces, "a kind of organ school", as Mendelssohn himself wrote to the publisher. Johann Sebastian Bach composed into his six Trio Sonatas "the sum of all the difficulties that a teacher wants a gifted pupil to master" (Marie-Claire Alain); written between 1727 and 1732, they remain a touchstone for every organist to this day. A good hundred years later, Mendelssohn created his Organ Sonatas - among other things, the independent and prominent pedal treatment (each movement features a pedal solo) and the frequent playing on several manuals challenge the player in the same spirit. Anything but monotonous studies, the results in both cases were masterpieces of the highest order.