Diminished Seventh Chords - Theory

Diminished seventh chords are very useful in jazz.  They add a bit of tension to a chord progression and resolve nicely to the next chord.

Diminished seventh chords are made up of 4 tones, the root, flatted 3rd, flatted 5th and doubled flatted 7th of the major scale.

Let's build a CDim7 chord.  Here's a C major scale:

root  C
2      D
3      E
4      F
5      G
6      A
7      B

So, a CDim7 chord would consist of C Eb Gb Bbb. Bbb is the same tone as A.

There are a couple of ways I look at a diminished seventh chord.

A diminished seventh chord is a dominant seventh chord with the 3rd, fifth and flat 7th all flatted.  Thus the Bbb.

My favorite way to think about a diminished seventh chord is to take a dominant seventh chord and sharp the root.

C Dominant Seventh     = C E G Bb
C# Diminished Seventh = C# E G Bb

Let's look at the intervals between the tones of a diminished 7th chord.

On the guitar an interval of a half step is is one fret difference between two tones.

root - flatted 3rd = 3 half steps = minor 3rd

flatted 3rd - flatted 5th = 3 half steps = minor 3rd

flatted 5th - double flatted 7th = 3 half steps = minor 3rd

double flatted 5th - root = 3 half steps (frets) = minor 3rd

Since the intervals between tones are all 3 half steps, any tone in the chord can be its name!

CDim7   = C Eb Gb Bbb (Bbb = A)
EbDim7 = Eb Gb Bbb Dbb  (Dbb = C)
GbDim7 = Gb Bbb Dbb Fbb (Fbb = Eb)
ADim7    = A C Eb Gb