The (originally Traité de l'harmonie réduite à ses principes naturels ) is a monumental 1722 work by French composer and theorist Jean-Philippe Rameau . Often cited as the foundation of modern Western music theory, it shifted the focus of music from horizontal melody to vertical harmony , establishing the mathematical and functional rules for tonality. Core Theoretical Contributions
: Introduces the practical application of his theories, explaining the fundamental bass, modes, and the rules governing chords. Treatise on Harmony
: Explores the physics of sound, focusing on string ratios, intervals, and the derivation of chords from a single "source" sound. The (originally Traité de l'harmonie réduite à ses
: He was the first to formalize that a chord like is the same entity as explaining the fundamental bass