Interchangeable Diminished 7th Chords

What is a diminished 7th chord?

First, it helps to know some basics about chord construction. Briefly, the standard chords are built from notes that are thirds apart. The choice to use major thirds and minor thirds determines the tonality of the constructed chord. If we pick a root note, then a note a major third above, and another note a minor third above that, we've constructied a major chord (we end up with the root, the major third, and the perfect fifth). If we use a note a minor third above the root, and a second note a major third above that, we've constructed a minor chord (root, minor third, perfect fifth). By adding a fourth note on top, we create seventh chords: choose a root note, a second note a major third above the root, another note a minor third above that, and a fourth note a minor third above that, we get a dominant seventh chord (root, major third, perfect fifth, minor seventh).

A diminished seventh chord comes from using all minor thirds. That is, we choose a root note, then a note a minor third above that, another note a minor third above the that, and then a note a minor third above that. So, a diminished 7th chord is made up of the root (r), the minor third (b3), the diminished fifth (b5), and the diminished seventh (bb7).

(Note that the diminished seventh (bb7) is NOT the flatted seventh (b7) but is a half-step lower... the diminished seventh (bb7) is the same as the major sixth (6)).

For example, a Dbdim7 chord is Db, E, G, and Bb. Notice that the root note (Db) is also a minor third up from the diminished seventh (Bb)... this is important in the next section.

Take home messages:

• Diminished seventh chords contain the root, the minor third, the diminished fifth, and the diminished seventh (e.g., Dbdim7 is Db, E, G, Bb).
• Two useful fingerings for Dbdim7 are x4535x and xx5656 (fret positions for strings, low to high (EADGBe)).

Substituting diminished 7th chords

There are a number of places that one can use a dim7 chord as a substitute for another chord. For example, in a ii-V-I jazz progression, a dim7 chord can be substituted for the Imaj7 chord (a half step above the I) as a passing chord between the I and ii. However, it turns out that dim7 chords can easily be substituted for each other by using a very simple trick. Anytime you play a dim7 chord, you can freely substitute any other dim7 chord whose root is 3, 6, 9, 12, 15, 18, or 21 (any multiple of 3) frets above or below the original chord's root.

Why? As described above, dim7 chords are built entirely out of minor thirds… for example Dbdim7 contains the root Db, the minor third E (a minor third above Db), the diminished fifth G (a minor third above E), and the diminished seventh Bb (a minor third above G). (Again, note that the root Db is also a minor third above the diminished seventh Bb).

So, when you slide that Dbdim7 up 3 frets (a minor third) to a Edim7, you take a chord that contains Db, E, G, and Bb and slide it to a chord that contains E, G, Bb, and Db… the same notes! Slide that Edim7 up three frets to Gdim7 and you have a chord with the notes G, Bb, Db, and E (same notes). Slide up three more frets and you get Bbdim7, which contains the same notes. (Hopefully, you can see that sliding down works the same way). Thus there are "families" of dim7 chords that are all interchangeable by virtue of sharing the same four notes. Effectively, the dim7 chords in a family can all be used as alternate voicings of one another. In practice, instead of playing Dbdim7 for four beats, you can play Dbdim7, Edim7, Gdim7, and Bbdim7 for one beat each without affecting the underlying harmonic structure and you get a nice bit of movement in your chord voicings.

Take home message:

• Move any dim7 chord three frets up or down the fret board to get a new voicing of the exact same chord!