The VII chord
Roman numerals in music refer to chords based on scale steps. The VII chord can mean different things depending on the actual musical key. In the key of C major, the VII chord is B.
Chords and intervals
The table shows how scale steps and chords are related in the key of C:
The seventh scales step is B, which result in the diatonic B diminished. The seventh note of a scale is also referred to as the subtonic.
Whereas the seventh degree is diminished in the context of the Major scale, the seventh degree is flatted major (bVII) in the Dorian mode and minor (bvii) in the Phrygian mode.
The viidim chord in all keys
The VII chord is identical with the seventh degree of the key, a list of the seventh chord would look like this:
C major key = B diminished
C# major key = C diminished
D major key = C# diminished
D# major key = D diminished
E major key = D# diminished
F major key = E diminished
F# major key = F diminished
G major key = F# diminished
G# major key = G diminished
A major key = G# diminished
A# major key = A diminished
B major key = A# diminished
So, for example, in the key of E, the viidim chord will be D#dim.
The VII chord in progressions
The viidim chord is clearly the least frequent in diatonic based progressions. In addition, dim and half-dim chords are normally not used as other diatonic chord but rather non-diatonically.
When the 3rd is omitted, the VII chord, can be used more freely. Such in the progression: G5 (I5) - F5 (VII5) - D5 (V5) - C5 (IV5) based on the pentatonic minor scale.
See also Chord theory | Music theory.