Chord III (Major Mediant)
The mediant triad built from the melodic minor scale is a major chord, because the interval between the root and 5th is a perfect 5th. This chord is the same as the tonic chord in the relative major key.
The mediant triad in A minor contains the notes C, E and G.
Major chord III is used frequently in minor keys, usually moving to or from chord VII (from the melodic minor scale) or chord iv.
This example of III moves from and to chord VII. It is from an Anglican hymn “Today O Lord a Holier Work”. The key is G minor, and chord III is Bb major.
Chord iii (Minor Mediant)
The chord built up from the 3rd degree of the major scale, or mediant note, is called the mediant triad, or chord iii.
The mediant triad is made up of the 3rd, 5th and 7th notes of the scale. The submediant triad in a major key is a minor triad.
The mediant triad in C major contains the notes E, G and B. It is a chord of E minor.
Chord iii is a secondary chord. Chord iii is relatively rare in Classical styles and is most likely seen
- in a progression moving to chord IV, ii or vi, or
- used as a at a point of modulation (key change).
Here is an example of chord iii used in a hymn. The progression is iii-IV. The key is A major, and iii is a chord of C# minor.
Here is an example of chord iii used as a pivot chord, modulating from A major to B minor. C# major is chord iii in A major, but chord ii in B minor. This extract is from Chorale 169 by JS Bach.
Chord III+ (Augmented Mediant)
The mediant triad built from the harmonic minor scale is an augmented chord, because the interval between the root and 5th is an augmented 5th.
The mediant triad in A minor contains the notes C, E and G#.
Augmented chords are written in capital Roman numerals, with a plus sign. They comprise two major 3rds, on top of each other.
Because the chord is very dissonant, it is not used very much in earlier styles, but became more common in 20th century music and later.
It is best to avoid using III+ if you are writing harmony exercises, because these are usually based on an earlier style of harmony, commonly used in the 18th century, where the augmented chord is not typically used.
Here is a rare example of III+ in a Bach Chorale (no. 11). This section is in D minor, and chord III+ is F augmented, F-A-C#. Notice that, as with diminished chords, the 3rd of the triad is doubled.
