Jazz harmony is the theory and practice of how chords are used in jazz music. While jazz bears certain similarities to other practices in the tradition of Western harmony, such as many chord progressions and the incorporation of the major and minor scales as a basis for chordal construction, other characteristics such as modality, syntax and phrase structure separate it from Western tradition. In jazz, chords are often arranged vertically in major or minor thirds, although stacked fourths are also quite common. Also, jazz music tends to favor certain harmonic progressions and includes the addition of tensions, intervals such as 9ths, 11ths, and 13ths to chords. Additionally, scales unique to style are used as the basis of many harmonic elements found in jazz. Jazz harmony is notable for the use of seventh chords as the basic harmonic unit more often than triads, as in classical music. In the words of Robert Rawlins and Nor Eddine Bahha, "7th chords provide the building blocks of jazz harmony."
The piano and guitar are the two instruments that typically provide harmony for a jazz group. Players of these instruments deal with harmony in a real-time, flowing improvisational context as a matter of course. This is one of the greatest challenges in jazz.
