How did Jean-Philippe Rameau explain the structure of chords?

In his influential treatise Traité de l’harmonie (1722), the French composer Jean-Philippe Rameau established a foundational theory of harmony based on the concept of the root, or fundamental note, of a chord.1

Rameau argued that all harmony is derived from this root, with other notes placed a third and a fifth above it to form a triad—the basic chord type of the common practice period. He explained that these notes can be arranged in either close or open positions across different octaves.1

Furthermore, Rameau introduced the concept of chord inversion, noting that a triad can exist in its root position or in rearrangements where the third or fifth serves as the lowest, or bass, note. While later theorists noted that these inversions lack the stability and finality of root-position chords, Rameau’s systematic approach remains the basis for the study of common-practice harmony today.1