Diminished seventh chord theory is simple you stack minor thirds on top of each other. That’s it.
In the major scale the diminished triad made from the 7th note becomes a minor 7th with a flatted 5th and not a diminished 7th like you might think. The 7th is actually a 6th from the root.
They used to teach you that years ago even in the Mel Bay Books which were the big authority on guitar back in the 60’s
This chord can come from the 7th note of the harmonic minor scale.
It can also be made from the diminished scale. The diminished and the diminished 7th are the only chords that can be made from the diminished scale.
There may be other sources that I’m not aware of but we’ll use the diminished scale to build this chord.
You have to understand scales before you can understand chord building theory. There are many scale sources for chords but most you will learn from the major scale.
Here are the diminished scales. There’s only two of them.
I put the chromatic scale there to stop confusion. This scale contains all the notes.
Some prefer to think there is only one scale you just start on the second note of the whole dim scale.
I’ve seen these called dominant scales because they are used over dominant chords
The first is called a whole diminished scale because it starts with a whole tone-two frets apart.
The second is called a half diminished scale because it starts with a half tone-one fret away.
Diminished Seventh Chord Theory
Making the Chord
Diminished seventh chord theory consists of stacking minor thirds together.
If you don’t understand what minor thirds are check out the page below then come back.
Here are the diminished scales with numbers.
Whole Diminished Scale
Half Diminished Scale
This chord will be made using the formula below
1-3-5-7 – From Diminished scale above, not the Major scale
If this was a major scale the A would be a 6th and not a 7th.
In the major scale the formula would be=
P.S. You may see where they call the 7th chord in the major scale a diminished 7th chord. They used to use this chord instead of the minor flat 5 chord. Old school.
Diminished 7th Chords
To complete your diminished seventh chord crash course here are some chords to use.
These chords are all the same chord.
Diminished seventh chord theory. All these chords have 4 names, one for each note tone.
They also repeat themselves every minor 3rd or 3 frets, the same distance we used to build the chord.
I hope you found this page useful.