The Mediant Harmonizing Mi in the Bass

Scale degree 3̂ (mi, the mediant) in the bass most commonly appears as the bass of I⁶ — the tonic triad in first inversion — which is one of the most frequently used harmonies in tonal voice leading. I⁶ functions as a tonic-prolonging chord: it shares all three pitches with root-position I but places mi in the bass, allowing the bass line to move by step while the harmony remains in the tonic zone. The mediant can also be harmonized by the mediant triad (iii or III), though this chord requires more care.

The mediant triad (iii in major) contains the leading tone (7̂, ti) and the supertonic (2̂, re) in addition to 3̂ (mi). Because it shares two pitches with V (the dominant triad), iii can sound like an incomplete dominant. Composers typically use iii in a passing or neighbor role — for example, connecting I and IV with a stepwise bass I–iii–IV — rather than as a structural harmony. Its ambiguity between tonic and dominant function means iii rarely appears at cadences, where harmonic clarity is paramount.

In minor keys, III (the major mediant, built on the flat 3̂) is a more stable and distinctive chord because it is the relative major — a full major triad with clear tonal identity. III in minor can move toward VI (the submediant) or toward VII in modal contexts, and it sometimes appears at half-cadence-like resting points. Whether in major or minor, harmonizing mi in the bass demands clarity of the surrounding voice leading to confirm whether the chord is functioning as a tonic first inversion, a passing mediant, or a structural mediant triad.