The diminished 7th chords are quite uncommon, but will often function as passings and is probably most seen in jazz progressions. If you are interested in the theory, the dim7 chord consists of a flatten third, a flattend fifth and a double flattened seventh besides the root. If we compare the structure of a C major 7th and a C diminished 7th the notes are C - E - G - B (Cmaj7) and C - Eb - Gb - Bbb (Cdim7). A diminished 7th chord can be written with an abbreviation "dim7" (Cdim) or a symbol "º7" (Cº7).
The dim 7th is a four-note chord in comparison with the triad dim chord.
The easiest way to learn dim7 chords is to memorize movable shapes. The lowest note is the root (see list below for full overview) in the diagrams with bass notes on the 4th and the 6th strings:
All diminished 7th chords with same movable shapes listed and written in short notation. Notice that the bass note is not always the root.
Cdim7 - X X 10 11 10 11
C# / Dbdim7 - XX2323
Ddim7 - XX0101
D# / Ebdim7 - XX1212
Edim7 - XX2323
Fdim7 - XX3434 / 123131
F# / Gbdim7 - XX4545
Gdim7 - XX5656 / 345353
G# / Abdim7 - XX6767
Adim7 - XX7878 / 567575
A# / Bbdim7 - XX8989
Bdim7 - X X 9 10 9 10 / 789797
A chord progression including this dim chord shape:
D - D#dim7 (XX1212) - Em (XX2000)
The dim 7th is built with the formula 1-b3-b5-bb7.
Notice that the dim7 chords are symmetrical. All inversions of the chord can be played with the same shape. This can be seen by comparing the following diagrams:
Dim7 chords with identical notes
This circumstance results also in that many dim7 chords share the exact same notes (so-called enharmonic chords). For example, Adim7 will use the same notes as Cdim/A, A#dim7 will use the same notes as C#dim/Bb and so on. Below is a larger comparison of this with names and short notation:
Ddim7 = Fdim7/D = Abdim7/D = Bdim7/D - XX01010
Edim7 = Gdim7/E = Bbdim7/E = C#dim7/E - XX2323
Fdim7 = Abdim7/F = Bdim7/F = Ddim7/F - XX3434
Gdim7 = Bbdim7/G = C#dim7/G = Edim7/G - XX5656
Adim7 = Cdim7/A = D#dim7/A = F#dim7/A - XX7878
Dim7 chords (closed and open positions)
The diagram that shows D#dim7 can also be written as D#dim7 and as slash chord: Cdim7/Eb.
With root note on the 6th string:
Adim7 - 5X454X
Chord progressions with dim chords
The typical function of a dim chord in sequences is as a chromatic passing chord:
Cmaj7 - C#dim7 - Dm7 - G7
F#7 (XX4320) - Edim7 - Dmaj7
Gm - Edim7 - F
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