Many of J.S. Bach's most beloved keyboard compositions were intended partly as instructional material. The six Trio Sonatas for Organ, BWV 525-530, may be such pieces; they are thought to have been incorporated into the lessons that Johann Sebastian gave to his son Wilhelm Friedemann. The six sonatas were composed close to the beginning of Bach's life in Leipzig. Unlike some of the others, which make use of genuine trio sonata music earlier composed for three players, the first of them, the Trio Sonata No. 1 in E flat major for organ, BWV 525, is apparently a wholly original work.
The process of condensing the by-then venerable trio sonata medium into music for a single keyboard player -- with the three original voices assigned to two manuals and the pedals -- was not accomplished in a single bold step. Bach's Three-Part Inventions of the early 1720s draw heavily on trio sonata idioms, as do several other keyboard works (the B minor Prelude in the first volume of the Well-Tempered Clavier being a key example). And we must also remember that while living in Cöthen in the early 1720s Bach had condensed the trio sonata texture for two players (e.g. the Six Sonatas for Violin and Harpsichord, BWV 1014-1019). Still, BWV 525 is something striking and new: a full-fledged chamber sonata for a single player, and probably the first of the organ trio sonatas to be composed.
Even so, there is very little else that is truly new about the music of BWV 525 -- the score might easily be played by two instruments and basso continuo, and very few listeners would be aware that it is in fact not an authentic trio sonata. The opening of the first movement is built by way of the normal upper-voices imitation, to which the bass voice adds "walking" eighth notes. The astute listener or player will certainly notice that the manner of the movement's active sixteenth notes owes as much to the Baroque concerto as to the Baroque sonata (as, indeed, does the late Italian Baroque three-movement format of BWV 525), but the fusion of sonata and concerto styles is something that we notice time and again throughout Bach's chamber sonatas.
The C minor Adagio is in a true binary form whose rhythms occasionally make quasi-siciliano shapes. The Allegro finale jumps forth in 3/4 time; the movement again falls into two halves, and at the start of the second half Bach turns the main subject of the movement -- a series of leaping eighth notes and some consequent sixteenth note runs -- upside down.