Instructive editions: still frequently used in music teaching; rejected by musicology and by "serious" musicians as textually unreliable; silently adopted by many students and sometimes consulted by accomplished musicians... Although they are commonly found both on music shop shelves and on the music stand of many performers, they are rarely studied, discussed and analysed. This book studies the phenomenon of instructive editions from a plurality of viewpoints: what are they? which problems are posed by their use? how and how much are they used today? how, when and why were they born? And, most important of all: what can they tell us as concerns past performance practice? Indeed, instructive editions become formidable tools for the study of past performance, being the distillation of their editor's concept of the work. Their usefulness for studies in performance practice is shown through the analysis of Italian editions of Bach's Well-Tempered Keyboard, an iconic work in keyboard literature and an all-time favourite in performance and teaching. The author's personal experience as a concert pianist and teacher gives a further practical insight into the processes of music making.