The III chord
Roman numerals in music refer to chords based on scale steps. The III chord can mean different things depending on the actual musical key. In the key of C major, the 3-chord is E minor and in the key of A minor the 3-chord is a C major.
Chords and intervals
The table shows how scale steps and chords are related in the key of C:
The third scales step is E, which result in the diatonic E minor. The third note of a scale is also referred to as the mediant. Since minor chords are written with small letters, Em is written as iii (sometimes is IIIm used).
Whereas the third degree is minor in the context of the Major scale, the third degree is flatted major (bIII) in the Dorian mode. The following table is based on the C Dorian:
It's possible to find additional contexts in which a major third degree is actual, for example, the third degree is E major in the C Lydian #5 mode.
The III chord in all keys
The III chord is identical with the third (major) degree of the key, a list of the third chord would look like this:
C major key = E major
C# major key = F major
D major key = F# major
D# major key = G major
E major key = G# major
F major key = A major
F# major key = A# major
G major key = B major
G# major key = C major
A major key = C# major
A# major key = D major
B major key = D# major
So, for example, in the key of F#, the III chord will be A#.
The iii chord in all keys
A similar set of relationship can be seen for the minor keys:
A minor key = Cm
A# minor key = C#m
Bm minor key = Dm
Cm minor key = D#m
C#m minor key = Em
Dm minor key = Fm
D#m minor key = F#m
Em minor key = Gm
Fm minor key = G#m
F#m minor key = Am
G minor key = A#m
G# minor key = Bm
So, for example, in the key of F minor, the iii chord will be G#m.
The iii, III and bIII chords in progressions
The iii chord is more common than the III in chord progressions. To use it after the I chord, can create a sentimental feeling and this change is common in ballads. Such as the progression C (I) - Em (iii) - F IV) - Em (iii).
Another example with iii could be C (I) Em (iii) - F (IV) - G (V). The same progression with extended chords: Cmaj7 (Imaj7) - Em9 (iiim9) - F6 (IV6) - G7 (V7).
The iii could on the other hand turn into III when harmony is borrowed from a related key. The diatonic sequence C (I) - Em (iii) - Am (vi) - G (V) could be transformed to C (I) - Em (III) - Am (vi) - G (V).
Another example is when the a bIII chord is borrowed from a related key. The diatonic sequence C (I) - Em (iii) - F (IV) - G (V) could be transformed to C (I) - Eb (bIII) - F (IV) - G (V). Another progression involving bIII together with bIV is A (I) - D (IV) - A (I) - G (bVI) - D (IV) - C (bIII).
The progression Gm7 (vi7) - Dm7 (iim7) - Ebmaj7 (bIIImaj7) is an example based on C Dorian.
See also Chord theory | Music theory.