Mediant

The middle between tonic and dominant

The word goes back to the Latin “medius”, the middle: in the stacking of thirds I – III – V the third degree lies between tonic and dominant. In its section on substitutes the page Diatonic chords says that the mediant iii (Em) is closely related to the tonic and colours it. Unlike the tonic relative (Tp) and the subdominant relative (Sp) it has no function name of its own there. In function theory, Em in C major is called, depending on context, dominant relative (Dp) or tonic counter-chord (Tg).

The table shows the diatonic chords in C major with function and church mode, as they are given on the page Circle of fifths. Em is a minor chord and belongs to the church mode Phrygian, which begins on the third degree.

How it sounds

Em shares two notes with the C major triad, namely E and G. The change C → Em therefore changes only one note, C becomes B, and sounds soft as a result without leaving the key. The listening examples show the same effect in a familiar sequence.

Third-related chords

In the Romantic period the word “mediant” is taken more widely: it denotes chords whose roots lie a third apart. Besides the diatonic mediant (C → Em) there are chromatic mediants, which do not lie in the key and share only one note with the starting chord, for example C major to E major. They open up surprising colours and often weaken the functional connection. That is the general textbook definition and goes beyond the page Diatonic chords.

Mediant and mode

The church mode on the third degree is Phrygian. The article The modes through familiar pieces describes Phrygian as minor with a lowered second degree: in E Phrygian there is an F instead of F♯, and this semitone above the tonic is the most striking feature. The notes of E Phrygian are exactly the white keys, so those of C major; Em is its tonic triad. In major-minor music the mediant mostly appears as a coloured passing chord, not as the target of a cadence.