The dim (diminished) chords are quite uncommon, but it is still good to be familiar with this chord category. If you play a dim chord you will hear that these chords lack harmony and this is because of the structure of the notes.
A common function for these chords is to be used as a chromatic transposition chord, like an in-between chord, also called a passing chord – this is most common in jazz. If you are interested in the theory, the dim chord consists of a flatten third and a fifth besides the root. If we compare the structure of a C major and a Cdim the notes are C - E - G (C major) and C - Eb - Gb (Cdim). A diminished chord can be written with an abbreviation "dim" (Cdim) or a symbol "º" (Cº).
The easiest way to learn dim chords is to memorize two movable shapes. Below you can see the shapes for dim and dim7. The lowest note is the root (see list below for full overview).
All diminished and diminished 7th chords with same movable shapes listed and written in short notation.
Cdim - X 3 4 5 4 X
Ddim - X 5 6 7 6 X
Edim - X 7 8 9 8 X
Fdim - 1 2 3 2 X X
Gdim - 3 4 5 4 X X
Adim - 5 6 7 6 X X
Bdim - 7 8 9 8 X X
Cdim7 - X X 10 11 10 11
Ddim7 - X X 0 1 0 1
Edim7 - X X 2 3 2 3
Fdim7 - X X 3 4 3 4
Gdim7 - X X 5 6 5 6
Adim7 - X X 7 8 7 8
Bdim7 - X X 9 10 9 10
Notice that the root note is the same as the bass note for all versions.
The chords including C#/Db, D#/Eb, F#/Gb, G#/Ab and A#/Bb are neglected, but you can easily find the right chord shapes for these. See fretboard for more information.
Dim chords (closed and open positions)
None for the moment in this category
Concerning the "non-movable" open and closed dim chords, in some cases an alternative bass note is used for making the chords easier to play. The diagram that shows Cdim is more correctly written as Cdim/Eb. The diagram that shows Edim includes a Db note, which is not fully correct, but is a practical solution in this case.
More dim chords in open positions.
A#dim / Bbdim - X1X020
More dim7 chords with movable shapes.
With root note on the 6th string:
Adim7 - 5 X454X
With root note on the 5th string:
Ddim7 - X5656X
Chord progressions with dim chords
The typical function of a dim chord in sequences is as a passing chord. In other words, you often don't stay long on a dim chord.
Gm - Cm - D - Gm - Cdim - Gm
Ab - Dbm - Adim
Back to chord types