harmony
The combination of simultaneously sounded notes to produce chords and chord progressions.
In Depth
Harmony is what happens when two or more notes sound together. It encompasses chords, chord progressions, voice leading, and the overall vertical dimension of music. While melody is the horizontal element — notes moving through time — harmony is the vertical element, creating depth and colour.
Western harmony is built on the relationship between tension and resolution. Dissonant chords create tension that resolves to consonant ones, and this interplay drives music forward. The study of harmony begins with simple triads and progresses through seventh chords, secondary dominants, borrowed chords, and eventually the extended and non-functional harmonies of jazz and modern classical music.
Western harmony is based on the overtone series, a natural acoustic phenomenon. The first five overtones of any note produce a major chord — harmony is literally built into physics.