In music, the sound of two or more notes heard simultaneously. In a narrower sense harmony refers to the extensively developed system of chords and the rules that govern relations between them in Western music. Harmony has always existed as the vertical (the relationship between simultaneous melodic lines) aspect of older music that is primarily contrapuntal; the rules of counterpoint are intended to control consonance and dissonance, which are fundamental aspects of harmony. However, the sense of harmony as dominating the individual contrapuntal lines followed from the invention of the continuoc. 1600; the bass line became the generating force upon which harmonies were built. This approach was formalized in the 18th century in a treatise by Jean-Philippe Rameau, who argued that all harmony is based on the root or fundamental note of a chord. Tonality is principally a harmonic concept and is based not only on a seven-note scale of a given key but on a set of harmonic relations and progressions based on triads (three-note chords) drawn from the scale.