Rod Stradling

The Accordion Pages


The mystery of the diminished chord

What does that last row of buttons on the left side of the accordion do? They are called the diminished 7th row but what does this mean?

You can download this music as a .pdf from here.

To start, let's construct a C diminished 7th chord starting from C major.

What is a diminished chord?

C major is C, E and G. C to E is a major 3rd interval (4 semitones) and E to G is a minor third interval (3 semitones).

To build a diminished chord you need 3 notes, starting from the fundamental (in this case, C) separated from each other by a minor third. C diminished is C, E flat and G flat.

The diminished 7th chord adds a further minor third to the list so C diminished 7th is C, E flat, G flat and A.

But remember that the accordion only plays 3 notes at a time on the chord buttons (more >>) and so G flat is left out, leaving the "accordion C diminished 7th" as C, E flat and A.

Amazingly, these 3 notes are an inversion of A diminished, which we would normally describe as A, C and E flat.

So, the accordion C diminished 7th button actually plays A diminished.

The next thing to recognise is that, whilst there are 12 different diminished chords, there are only 3 diminished 7th chords.

If you add 4 minor thirds together you get back to the same note an octave higher. So, if you look at the stave below you can see that you can only have 3 diminished 7th chords before you start to make inversions of the ones you have already made.

There are only 3 diminished 7th chords

Now here is where it gets really confusing; Why do we call that row of buttons the diminished 7th row when it is actually a row of diminished chords relating not to the fundamental bass button in that row but to the 6th of its major scale (in the case of the C row, not C but A)? Well, I don't know the answer to this so I simply live with the fact that that's the way it is. It does explain why, when you play a diminished 7th button and then move up 3 buttons and expect it to sound the same, it doesn't. The C diminished 7th button plays C, E flat and A but the A diminished 7th button plays A, C and G flat. On the other hand, both buttons represent A, C, E flat and G flat diminished 7 but in each case with one note of the chord missing. This means that, when the music calls for a particular diminished 7th chord you don't have to reach too far. On the other hand, if you are asked to play a diminished chord, you have to be carefull what you play. There is some relief to be gained by realising that, often, diminished and diminished 7th chords are regarded as interchangeable. Bob van der Poel from Canada reminded me that the fact that three notes sound makes them a similar volume to the other chord notes.

But..... It does not end here! Andy Fielding from Western Canada got in touch to point out that, when you play a diminished button, you (nearly) get a minor 6th chord. For example, the C diminished chord consists of C, E flat and A and Cm6 consists of C, E flat, G and A. Depending on where the chord is used, your mind decides whether to hear the missing note.

Once you have got over your headache, you might want to read the page on the use of diminished chords.

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