The strange mathematical anomaly of the Diminished 7th chord.
They are some very special chords. [Intermediate Lesson]
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Intermediate Lesson
The Diminished Concept
Let's take a look at a chord that is really only useful in context, in a chord progression - but, by itself it doesn’t sound very good. The diminished 7th chord is a low percentage chord used in music to bridge a specific gap in a chord progression.
Where is this gap?
Well, you can sometimes use it between the "I" and the "ii" chords.
For example:
C, C#°7, Dm, G7 or CM7, C#°7, Dm7, G7.
Another example:
G, G#°7, Am, D7 or GM7, G#°7, Am7, D7.
It can also be used between the "V" and the "vi" chords.
For example:
A, A#°7, Bm, G, A, D or A7, A#°7, Bm7, GM7, A7, DM7
Generally speaking, you probably won't begin or end too many songs with this chord. It is what's known as a "transition" chord. You transition through this chord, you don't start or stop on it.
Why? Because, out of context in a chord progression, it just doesn't sound very good by itself. This is because of the strange mathematical anomaly that is used in the structure to make the chord.
Diminished Triad and the Diminished 7th structures.
The basic diminished triad requires three notes:
“The root note, a minor 3rd and a diminished 5th.”
To make a diminished 7 chord, you need four notes:
“The root note, a minor 3rd, a diminished 5th and a "double-flatted” 7th.”
What is a double-flatted 7th?
In the major scale, the seventh note is the Major 7th. If you lower that note one-half step, it becomes a "dominant" or flatted 7th. This is used in blues and jazz chords. Lower it another half-step; it takes the place of the Major 6th note in the scale, but it is not called a Major 6th, rather a double-flatted 7th.
The Half-Diminished 7th chord.
Without confusing you too much further, there is also a "Half-diminished 7th" chord, which is somewhat counter-intuitive as it only requires the root note, a minor 3rd, a diminished 5th and a dominant 7th". Most musicians simply refer to this chord as a minor 7b5.
In this diagram, we can see that there are white, yellow, red and blue dots to mark the fingering positions to make the Diminished 7th chord. Notice, they are all the same shape. Here is where it gets a little weird. In the structure of the °7 chord, each note can be thought of as a "root" note. That is because each position is a minor 3rd interval from the next note.
In this diagram, we place the notes on each dot to signify its position in each of the chords. Notice, each chord has the same exact four notes. That means each chord could be looked at as the exact same chord but with notes in alternating interval placements. In other words,
E°7 is the same chord as Bb°7
Bb°7 is the same chord as Db°7
Db°7 is the same chord as G°7
G°7 is the same chord as E°7
Weird, huh? So, in all actuality, if there are 12 notes used in music, that means the exact same chord can be used for all four chords, in separate three groups. In other words, move all four of these chord shapes up or down one-half step to find the other eight chords.
I told you it was a weird chord. I guess that is why it is used so infrequently in pop music. It is however, used a great deal in jazz and blues music.
I hope that helps you make better sense of the use of diminished 7th chords.