2. Seventh Chords
A seventh chord is a triad chord with an added seventh interval above the root. Many composers are fond of seventh chords because of the unclearness and subtle color of the sound added by a dissonant interval from the root to the seventh, a “7th.” Because of this dissonance, a seventh chord usually requires a resolution to release the tension. A seventh note in a seventh chord is usually resolved to a step below. This inclination follows not from rules but from humans’ perception and preference.
The inclination (requiring a resolution) of seventh chords is common in Western classical music and has been one of the major principals in the tonal system. But this tendency may not be appreciated in other cultures or contemporary music, as people’s perceptions of consonance and dissonance are different and changing. In fact, many young American students do not hear a tritone (augmented 4th) as a keen dissonance, even in a tonal texture.
Here are a useful table and exercises on various seventh chords:
CH7) Exercise for Seventh Chords