René Descartes' early writing, the Compendium Musicæ, is presented in an annotated new edition with a new translation, lemmatised index and concordance of selections. It is aimed not only at musicologists, philosophers and mathematicians, but also at anyone interested in the history of science. It is little known that Descartes revisited his music theory in a late letter and developed his design of a diatonic scale into that of an enharmonic scale. Rolf Ketteler has also included this in this edition, which provides a new perspective on the explanations of the Compendium Musicæ.Translated with www.DeepL.com/Translator (free version)