The pieces chosen for the discussion, the Fantasie and Sonata in C minor, K. 475/457, are especially well documented in terms of their sources: in addition to the composing autographs and the first edition, there is the so-called dedication copy, a manuscript written by a copyist that was given to Maria von Trattner, to whom the Sonata was dedicated. The discussion will include a close examination of these three primary sources, several editions published during the late eighteenth and the nineteenth centuries (including that edited by Johann Anton André, who owned the autographs at the time), and representative recorded performances from throughout the twentieth century. Thus, the editorial and performance history of the Fantasie and Sonata will be traced from the time of the works' creation to today, revealing interpretive changes through time and providing a solid basis for a modern interpretation of the works based on Mozart's original notation and late-eighteenth-century performance conventions.