Common-Tone Chords

Common-tone chords are chromatic harmonies that share a pitch with the tonic triad and appear as coloristic embellishments rather than functional harmonic pillars. The two most important are the common-tone diminished seventh (CT°7) and the common-tone augmented sixth (CT+6). Unlike secondary-function chords, they do not resolve to a new tonal area — they expand a single harmony by adding chromatic tension around it.

The common-tone diminished seventh (CT°7) is built on a chromatic neighbor pitch a half step above or below one member of the chord it embellishes. Most often it decorates I or I⁶, with the common tone held in an upper voice while the other voices move by half step. The diminished seventh resolves back to the same tonic harmony it preceded, making it a prolongation device rather than a progression. Because every note of a dim7 chord is equidistant (all minor thirds), the same spelling can embellish different triads simply by identifying which pitch serves as the common tone.

The common-tone augmented sixth (CT+6) works by similar logic: it retains a common tone from a tonic chord and resolves back to tonic rather than forward to the dominant. Like the standard augmented sixth chords, it contains an interval of an augmented sixth that expands outward to an octave on resolution. CT+6 chords appear less frequently than CT°7 but produce a particularly intense chromatic shimmer, especially when they decorate the tonic before a final cadence.

Both chord types are best understood as chords of prolongation: they intensify a single harmony rather than driving motion between harmonies. When analyzing, look for a stationary pitch in one voice and half-step motion in the remaining voices — that combination almost always signals a common-tone chord. Their chromatic voice-leading is smooth, and their effect on the ear is one of brief, expressive tension immediately melted away by return to the enclosing harmony.