Inversion

A chord is in root position when its root is in the bass. When a different chord tone is placed in the bass, the chord is inverted. For triads: first inversion places the third in the bass; second inversion places the fifth in the bass. Inversions do not change which chord it is — the root, quality, and Roman numeral label remain the same — but they alter the sound and voice-leading possibilities.

Seventh chords have one additional inversion: third inversion places the seventh in the bass. The four positions of a seventh chord are root position, first inversion, second inversion, and third inversion. Each inversion has a distinct sonority. Root position and first inversion triads are generally stable; second inversion triads (the 6/4 chord) are more dissonant and have specific uses such as the cadential 6/4 or passing 6/4.

Figured bass symbols compactly indicate inversion. A triad in root position carries no figures (or the implicit 5/3); first inversion is labeled 6 (shorthand for 6/3); second inversion is labeled 6/4. For seventh chords: root position = 7, first inversion = 6/5, second inversion = 4/3, third inversion = 4/2. These numbers describe the intervals above the bass, giving performers the harmonic content from just the bass line.