A chord is the atom of harmony. The classical triad (three pitches spaced in thirds) sounds bright in its major form and sad in its minor form; diminished and augmented chords create tension.
Seventh chords (four pitches) are the foundation of jazz and blues. Clusters (dense stacks of adjacent pitches) are used by modern composers to create noise or tension. Chords are the language a composer “speaks” to the listener: through them pass longing, joy, and mystical foreboding.
Chords are classified by the number of pitches (triad, seventh chord, ninth chord), by construction (tertian, quartal, cluster), and by their function in a key (tonic, subdominant, dominant). An inversion — when the bass is not the root but the third or the fifth — changes a chord's “weight” and colour without changing its content. It is the sequence of chords (the harmonic progression) that shapes the drama of most works, from a chorale to a pop song.
