Few theorists have had such in-depth command of the diverse disciplines relating to the history of music theory as François-Joseph Fétis. Superbly lucid and systematic, his Complete Treatise on the Theory and Practice of Harmony is one of the most important and influential documents of the nineteenth century, witness its twenty-one printings between 1844 and 1903. Drawing upon ideas set forth by writers such as Choron and Jelensberger, Fétis fleshed out a historical approach to the theory of tonality, formulating his famous categories of unitonic, transitonic, pluritonic, and omnitonic harmonic orders. The third and fourth of these categories involve the enharmonic reinterpretation of pitches and offer insights into the expanded modulatory practices of the nineteenth century. This excellent new translation is based on the ninth edition (1867), the last to be modified by the author, and includes the introduction to the ninth edition, the philosophical preface to the third edition, and Fétis's response to criticisms of his theory in a twenty-page addendum.