In their originality, concentration and breadth of expression, the Polonaises of Johann Bach’s first-born son, Wilhelm Friedemann, establish him as a composer worthy to be measured against his father. Arguably his most significant compositions, these eloquent, poetic and technically demanding works pioneered the expansion and ultimately the emancipation of the tonal system. The Sonata in D major, Wilhelm Friedemann’s “Opus One”, was the work with which he first presented himself to the public in published form. Both in its expressive aims and the technical demands it makes on the player, the work poses levels of difficulty very rarely seen in eighteenth-century keyboard music.