In our harmonic system, the fully diminished 7 chord (R ♭3 ♭5 ♭♭7) ‘first appears’ in the Harmonic minor. We saw that in A Harmonic minor, the vii chord is G♯o7. In all harmonic minor tone groups, the seven chord is a viio7.
The fully diminished 7 chord formula is R-♭3-♭5-♭♭7 (3-3-3-3 – all minor 3rds – the double flat 7 is also known as the 6). What this means is that once we build this chord, and move it 3 frets up or down, it is the same chord. If a chord is built with all minor 3rds, and we move it 3 frets, all of the relationships are constant. And this means that every tone in the chord is the root, is the flat 3rd, is the flat 5, and the double flat 7.
Another important point is that there are only 3 of these chords. If we were to build the same type of chord starting on the next fret up from the first, this will be the 2nd, and 2 frets up, the 3rd. Once we move it up one more, we are back at the first.
There are only 3 serving all ’12’ keys.
We will be learning 3 voicings of the these chords (on 4 adjacent strings), then modifying each shape 4 times (lowering a tone). In doing so, we create 4 Dominant chords.