Mediants

A mediant relationship exists between two chords or keys whose roots are a third apart. In diatonic tonal music the mediant (iii or III) and submediant (vi or VI) are the standard third-relations, and they share two common tones with the tonic. Chromatic mediants extend this idea to include third-related chords that are not diatonic to the same key — for example, C major and E♭ major, or C major and A major. These chromatic mediant pairs share exactly one common tone and differ in mode quality (one major, one minor) or involve a mixture that puts them outside the same diatonic collection.

The defining traits of a chromatic mediant progression are: (1) the roots are a major or minor third apart; (2) both chords share at least one common tone; and (3) at least one chord is non-diatonic to the prevailing key. In C major, the diatonic mediant is E minor (iii) — but E major (III) is the chromatic mediant because it requires a chromatic alteration (G♯). Similarly, A major (VI in C major) introduces C♯ and changes the mode quality compared with the expected diatonic vi (A minor).

Chromatic mediants produce a characteristic sound of sudden harmonic color shift: one common tone ties the two chords together while the other voices move by half step or whole step. This smooth voice-leading makes the progression feel both surprising and inevitable. Schubert was a master of chromatic mediants; his music frequently moves between tonal centers a third apart (C major → A♭ major, for example) with no preparation other than the common tone.

When analyzing chromatic mediant progressions, label the chords with their Roman numerals and note the chromatic alteration (e.g., ♭VI, ♭III, III). Identify the common tone and trace how the remaining voices resolve. The progression's logic is voice-leading logic, not functional logic — there is no "dominant" preparing the arrival. Understanding mediants as voice-leading phenomena rather than root-motion phenomena aligns with the neo-Riemannian perspective: these are transformations of triads connected by minimal motion, not functional progressions driven by a tonal hierarchy.