The dim (diminished) chords are quite uncommon, but it’s 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 a between chord - which is most common in blues and 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 notes 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 "dim" (Cdim) or "º" (Cº).
The easiest way to learn dim chords is to memorize to shapes that are movable. Below you can see the shapes for dim and dim7 chords. The lowest note is the root (see table below for full overview).
All dim and dim7 chords listed with shapes written in numbers.
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 8 9 8 9
The chords of 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 open chords
None for the moment in this category
Concerning the open dim chord, in some cases is an alternative bass note used for making the chords easier to play. The diagram that shows Cdim is more correctly written as Cdim/A. The diagram that shows Ddim includes a C note, which is not fully correct, but is a practical solution in this case. The same is the case with the diagram that shows Edim since it includes a Db note, but once again it serves as a practical solution.
Chord progressions with dim chord
The typical function of a dim chord in sequences is as a passing chord. In other words you don't stay long on the dim chord.
Gm Cm D Gm Cdim Gm