Only recently has it become obvious that conductors' annotated scores and marked orchestral parts are of great cultural, historical and musical importance. In the not-so-distant past, these artefacts had something of an uncertain status with many either languishing unopened in libraries and family archives or simply being dispersed or discarded. With the help of institutions such as the Royal Academy of Music, Harvard University and the New York Philharmonic Orchestra this has begun to change with their extensive collections of these materials now being made available to scholars and musicians. This element examines the emergence of these artefacts as didactic and interpretative tools and explores the ways in which the performance styles of ten iconic conductors active in the nineteenth, twentieth and twenty-first centuries are reflected in their annotated scores and marked orchestral parts of Mozart's Symphony No. 41, K. 551 ('Jupiter').