Diminished 7 Chord Theory

This article is an introduction to the diminished 7th chord.

For many guitarists it's a relatively unusual chord, standing apart from the usual major, minor and seventh chords.

Jazz guitarists are more likely to use it regularly. It is found in many jazz chord progressions, and is commonly used as a chord substitution (i.e. it is used in place of another chord to change the harmony). More details on substitutions later in the lesson.

This article covers diminished 7 chord theory, how to play these chords on the guitar and how to use them in your own compositions.

Diminished 7 Chord Theory

Before we begin, here is a common way of playing a diminished 7th chord on guitar. Play it now to hear what the chord sounds like.

You'll probably recognize the sound: it's very common in many forms of music.

You can find another way of playing diminished 7th chords on guitar here.

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Diminished Triads

The diminished seventh chord is a diminished triad with an added seventh note.

Diminished triads span a diminished 5th interval. The first and second, and second and third notes are each a minor 3rd interval apart.

Diminished 7ths

By adding an additional note a minor 3rd above the top note of a diminished triad, a diminished 7th chord is formed.

The term 'diminished' is often used to refer to a diminished 7 (especially in jazz fake books).

The diagram below shows how a diminished 7 chord is constructed.

* The notation actually shows an augmented 2nd, but they’re ‘enharmonically equivalent’ (i.e. the same thing!)

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The Important Feature Of Diminished 7 Chords

One notable, and important, feature of diminished 7th chords is that each note is a minor 3rd away from the previous note.

Looking at a C diminished 7th chord, we see that the notes are:

C, E flat, G flat and A.

Why is this important?

Well, if you play the chord in 1st inversion - so that the E flat is the bass note - then the notes would be E flat, G flat, A and C.

(See this page for information on chord inversions.)

The notes in an E flat diminished 7th chord are: E flat, G flat, A and C.

Essentially the same chord!

The same thing occurs if you play the original chord in 2nd and 3rd inversions.

Basically, a C diminished 7th chord contains the same notes as E flat, G flat and A diminished 7th chords.

If you think about this, you will see that in fact there are only really three diminished 7th chords: those that contain either C, C sharp or D notes (or their enharmonic equivalents).

1: Chords containing C, Eb, Gb, A notes

2: Chords containing C#, E, G, Bb notes

3: Chords containing D, F, Ab, B notes

Another Interesting Feature Of Diminished 7th Chords

Another interesting feature of diminished 7th chords is that if you flatten any one of the notes in the chord, then the resulting chord is a 7th chord.

Use this knowledge to create interesting chord progressions and key changes in your own music.

Diminished 7th Chord Substitution

A popular way of using diminished 7th chords in jazz and other styles of music is to substitute a dominant 7th chord with a diminished 7th chord whose root note is a half step higher (e.g. swap a G7 chord for a G sharp diminished 7th).

The first chord progression below contains a normal 7th chord.

In the second progression the 7th chord has been substituted with a dim7 chord.

Play both progressions to hear the sound of the substitution; it gives the music a nice jazz feel. It also serves to pull the music back to the C chord, just as the G7 does. You can use this substitution in any kind of music.

Diatonic Diminished 7th Chords

The diminished 7th chord is not diatonic in a major key (see more about diatonic chords here).

Diminished 7th chords are constructed from harmonic minor scales, and are built up from the 7th degree.

Conclusion

The diminished 7th chord has an easily recognisable sound and is a useful chord to know. It occurs in many forms of music, and you should learn at least a couple of ways of playing it on guitar.

As we have found, diminished 7th chords have some interesting qualities. If you are aware of these you have some powerful tools for your rhythm guitar playing and composition.

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