Like Mozart, Mendelssohn was regarded as one of the finest pianists of his day but also maintained a life-long interest in the organ. In the same year as his first composition for piano (1820) came his first for organ, a sombre, slow-moving chordal Prelude in D minor. While he continued to learn the piano, and acquire early a formidably versatile technique under Ludwig Berger, his chief musical mentor, Carl Zelter, covered wider ground to include two of the formative influences on Mendelssohn’s later writing for the organ—a study of J S Bach and, from its first publication in 1821, the Choralbuch of Michael Gotthardt Fischer (1773–1829). Mendelssohn wrote a conservative set of variations on one of the tunes in this book, and Fischer’s own organ works, too, form a vital ingredient in the development of writing for the instrument in a new, flexible and dramatic yet idiomatic way. He was organist at Erfurt (also a Bach connection) and his virtuoso writing for both pedals and manuals is a hitherto underestimated force in the development of organ-writing in the early Romantic period. A W Bach (only a distant relation to the great Bach family) was another leading organist, and also composer, who directly influenced the young Mendelssohn, this time through a course of formal instruction.