The seventh is an “almost-octave,” just one semitone shy of closing the circle. That “almost” creates enormous tension.
The dominant seventh chord (containing a minor seventh) is the cornerstone of the classical cadence: it yearns to resolve to the tonic like a drawn bowstring longs to release. In jazz, seventh chords (major, minor, dominant with alterations) are the basic language of harmony. Without sevenths, jazz simply would not exist.
A distinction is drawn between the major seventh (a semitone short of the octave, sounding sharp and refined), the minor seventh (characteristic of the dominant seventh chord), and the diminished seventh. The skill of resolving sevenths — moving the dissonance to the nearest stable pitch — forms the basis of classical voice leading. In the Romantics and especially in jazz, seventh chords ceased to be merely “tension on the way to the tonic” and gained an independent colouristic value.
