Diminished 7th Chord

The Diminished 7th chord (which is sometimes referred to as the "Fully" Diminished 7th chord in order to differentiate between it and the Half-Diminished 7th chord) is the "lowest" chord arrangement possible for 7ths. Each note is stacked a minor 3rd above the previous note.

The mere mention of this chord scares many people, simply because they do not understand it. It is a very versatile chord, however, with a variety of uses (especially in chord progressions and substitutions). This amazing chord even occurs four times in the hymn The Old Rugged Cross, although most church pianists never realize it. We describe it as having a Diminished/Diminished construction in the Triad/Interval system.

As an example, let's form an E Diminished 7th chord.

1. A Diminished 7th chord is made up of a Diminished triad, and a Diminished 7th interval.
2. The name of the chord gives us the root, that is, E.
3. We know that an E Diminished triad is made up of E, G, and Bb.
4. The Diminished 7th Interval above E is Db.
5. Therefore, the notes E, G, Bb, and Db make up the E Diminished 7th chord.

E Dim 7th

Let us follow the same procedure again to form an Ab Diminished 7th chord.

1. A Diminished 7th chord is made up of a Diminished triad, and a Diminished 7th interval.
2. The name of the chord gives us the root, that is, Ab.
3. We know that an Ab Diminished triad is made up of Ab, Cb, and Eb.
4. The Diminished 7th Interval above Ab is Gbb.
5. Therefore, the notes Ab, Cb, Eb, and Gbb make up the Ab Diminished 7th chord.

Ab Dim 7th

If you didn't notice in the above example, it is quite common (though not always the case) for a Diminished 7th to have one or more double flats. Now we can add the Diminished/Diminished form of the 7th Chord to our chart, but we are not done yet. We still have one more type of 7th chord to cover.

Here is the chart as applied to a C chord: