Partly because Sor was himself such a classical guitar virtuoso—contemporaries considered him to be the best in the world—he made a point of writing didactic music for players of that instrument of all levels. His Twelve Studies Op. 6, the Twelve Studies Op. 29, the (24) Progressive Lessons Op. 31, and the (24) Very Easy Exercises Op. 35 have been widely played for two hundred years and are regularly reprinted. On the other hand, some of Sor's music, not least his popular "Introduction and Variations on Mozart's "Das klinget so herrlich"" Op. 9, is fiendishly difficult.