The II chord
Roman numerals in music refer to chords based on scale steps. The II chord can mean different things depending on the actual musical key. In the key of C major, the II chord is D. Since minor chords are written with small letters, Dm is written as ii (sometimes is IIm used).
Chords and intervals
The table shows how scale steps and chords are related in the key of C:
The second scales step is D. The second note of a scale is also referred to as the super tonic.
Whereas the second degree is minor in the context of the Major scale, the second degree is major in the Lydian mode, which the second table shows:
The II chord in all keys
The II chord is identical with the second (major) degree of the key, a list of the second chord would look like this:
C major key = D major
C# major key = D# major
D major key = E major
D# major key = F major
E major key = F# major
F major key = G major
F# major key = G# major
G major key = A major
G# major key = A# major
A major key = B major
A# major key = C major
B major key = C# major
So, for example, in the key of C#, the II chord will be D#.
For other chords types, a number or a sign is added. For example, IImaj7. In the key of A, IImaj7 will be Bmaj7.
The ii chord in all keys
A similar set of relationship can be seen for the minor keys:
A minor key = Bm
A# minor key = Cm
Bm minor key = C#m
Cm minor key = Dm
C#m minor key = D#m
Dm minor key = Em
D#m minor key = Fm
Em minor key = F#m
Fm minor key = Gm
F#m minor key = G#m
G minor key = Am
G# minor key = A#m
So, for example, in the key of E minor, the ii chord will be F#m.
The II and ii chords in progressions
The ii chord is the most common of the two. It's included in the ii - V - I progression that is fundamental in jazz. In the key of Bb, an example could be Cm7 (iim7) - F7 (V7) - Bbmaj13 (Imaj13). Another example involving the half-dim could be Bm7 (iim7b5) - E9 (V9) - Amaj7 (Imaj7).
Another simple example is I - ii - V, which in the key of G would be G - Am - D.
Since the second degree is minor in major scales, the II is quite uncommon. One example is D (I) - E (II) - A (V) - D (I), but this could rather than major be considered as based on the Lydian mode.
See also Chord theory | Music theory.