The seventh chord is formed by adding a fourth note to a triad at the distance of another third — creating the interval of a seventh between the outer tones. This seventh brings a dissonant tension into the sonority that seeks resolution, which makes seventh chords the “engines” of harmonic motion.
The most important of them is the dominant seventh, the main “magnet” that pulls the harmony toward the tonic. In classical music, seventh chords were strictly prepared and resolved. In jazz, however, they became the basis of the language: there the seventh chords (as well as ninth and eleventh chords) sound on their own, as stable colourful sonorities, and it is precisely their richness that defines the recognizable “flavour” of jazz harmony.
