How to form a diminished chord

A diminished chord has -like a minor chord- a minor 3rd interval from the first note (the root) to the second note of the chord. The difference with minor chords lies within the 3rd chord note as we will see below. Like with minor chords and major chords, we can have diminished triads and diminished 7th chords.

If you want to hear sound samples of the different chords, please refer to the lesson ‘What is a chord? How do different chords sound?’.

Diminished triads

A diminished triad is made of the root, the minor 3rd and the flattened 5th. So, in the case of the C diminished triad, this would be:   C   Eb   Gb

Another example is the G diminished triad: G   Bb   Db

The notation for a diminished chord is (in this case C diminished):     C O or Cdim. And in the case of G diminished: G O or Gdim.

Half diminished chords

As with major and minor chords, we can add the 7th. When we add the minor 7th to a diminished triad, we get a half diminished chord. Let’s see how that works in the key of C. The C diminished triad is C   Eb   Gb. The minor 7th in the key of C is the Bb, so C half diminished is: C    Eb    Gb    Bb

The notation for the C half diminished chord is: CØ or Cm7b5.

CØ is a nice and short notation, but Cm7b5 actually shows better what’s going on in the chord:

  • The ‘m’ stands for minor, since we have a minor 3rd
  • The ‘7’ stands for the 7th in the chord, in this case a minor 7th
  • The ‘b5’ stands for the flattened 5th in the chord

The G half diminished chord is: G   Bb   Db   F

So we can write G half diminished as: GØ or as Gm7b5

A half diminished chord can also be considered as a minor chord (but a minor chord with a flattened 5th).

Diminished 7th chords

Perhaps you noticed that a diminished triad is made of 2 stacked minor 3rd intervals. Look at the C diminished triad: from the root (C) to the minor 3rd (Eb) is a minor 3rd interval and from Eb to Gb is also a minor 3rd interval. Well, why not adding another minor triad? So, let’s do that!

What is a minor 3rd up from Gb? A minor 3rd consists of 3 semitones and 3 semitones up takes us to A. The only problem is that the 3rd note in the Gb minor scale cannot be an A.  Ab is the 2nd note, so the 3rd note must be written with the letter ‘B’ (see the rules in the major scale lesson). The only way we can do that, is with a double flat: Bbb (which is of course the enharmonic equivalent of A).

So, the C diminished chord is: C   Eb   Gb   Bbb

G diminished is a bit easier because it doesn’t contain any double flat: G   Bb   Db   Fb

The notation for diminished chords is as follows:

C diminished: C O7, Cdim7 or C O

G diminished: G O7, Gdim7 or G O

Even though diminished chords have a minor 3rd, they are, in contrast to half diminished chords, not considered as minor chords.

All the other diminished chords

Diminished 7th chords

And here comes the good news: there are only 3 different diminished 7th chords! Not 12, as was the case with minor and major chords (but this is only for diminished chords, not for half diminished chords since there are 12 different half diminished chords!). Only 3? Let me explain:

Look at the Eb diminished chord:

  • From Eb, up a minor 3rd to Gb
  • Then, from Gb, a minor 3rd up to Bbb
  • Finally, from Bbb, a minor 3rd up to Dbb (which is a C)

So, Eb diminished is: Eb   Gb   Bbb   Dbb (or C)

Compare this with the C diminished chord: C   Eb   Gb   Bbb

Even when not written totally the same (Dbb instead of C), the chords are exactly the same! And, indeed: a diminished chord of a note of the C diminished chord has the same notes as the C diminished chord itself. So this means that Cdim7, Ebdim7, Gbdim7 and Adim7 all are the same chord!

(Btw, notice that I wrote Adim instead of Bbbdim, because it would be a bit ridiculous to talk about the Bbbdim chord when we can simply say Adim.)

This enables us to finally make the table with the 3 different diminished chords:

Diminished chord: Chord notes:
C O7, Eb O7, Gb O7, A O7 C    Eb    Gb    A
Db O7, E O7, G O7, Bb O7 Db   E   G   Bb
D O7, F O7, Ab O7, B O7 D   F   Ab   B

Some remarks concerning this table:

  • Notice that the roots of the chords listed in the left column are the same as the notes of the diminished chords in the right column.
  • Instead of writing the correct notes for each individual diminished scale (like Bbb for Cdim), I wrote the easiest enharmonic equivalent (A instead of Bbb).
  • For the diminished chords with a black note root: I didn’t list the enharmonic equivalents, but you can find the notes of for example D# O7 at Eb O

Half diminished chords

Here’s the table with half diminished chords:

Half diminished chord: Chord notes:
CØ C   Eb   Gb   Bb
DØ D   F   Ab   C

 

EØ E   G   Bb   D
FØ F   Ab   Cb   Eb
GØ G   Bb   Db   F
AØ A   C   Eb   G
BØ B   D   F   A
DbØ / C#Ø Db   Fb   Abb   Cb  /  C#   E   G   B
EbØ / D#Ø Eb   Gb   Bbb   Db  /  D#   F#   A   C#
GbØ / F#Ø Gb   Bbb   Dbb   Fb  /  F#   A   C   E
AbØ / G#Ø Ab   Cb   Ebb   Gb  /  G#   B   D   F#
BbØ / A#Ø Bb   Db   Fb   Ab  /  A#   C#   E   G#

 

 

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Martin Cohen
 

Martin Cohen is a science and piano teacher. He is also a jazz musician and composer.

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