Besides the already mentioned categories, you should know also know of the half-diminished seventh chord. This chord is identical to the diminished seventh except for the seventh note that is a flat instead for a double flat (it could also be seen as a minor seventh with a lowered fifth). The half-diminished is more commonly written as m7b5 or m7(b5).
Comparing the Major Chord with the different diminished chords:
C: C – E – G
Cdim: C – Eb – Gb
Cdim7: C – Eb – Gb – A (Bbb)
C half-diminished 7: C – Eb – Gb – Bb
The B double flat (Bbb) in Cdim7 is used since the last tone is theoretically a major sixth.
Formula and steps
The formula for the dim triad chord is 1 - b3 - b5. There are three half steps to the 2nd note and three half steps to the 3rd.
Alternative chord symbols
Besides the dim chord symbol that is presented above, you may also find the alternative degree symbol (°), as in C°, being used occasionally. Half-diminished seventh chord could also be written with a symbol (ø), as in Cø.
Make use of
And the most important part: how and when to use diminished chords? One of the most common ways is to use them as passing chords. Passing chord can be seen as “in-between chords”, in other words: chords to land on for a short while to create some tension between chords with stronger relationship with the key.
Chord sequences that shows how dim chords can be utilized:
1. C Ebdim G7
2. G Bbdim D7
(The intervals are the same: I biii° V7.)
Examples including dim7:
1. Cmaj7 Dbdim7 Dm7
2. Emaj7 Fdim7 F#m7
(The intervals are the same: Imaj7 bii°7 ii7.)
Examples including half-diminished:
1. Am7 Bm7b5 E7
2. Em7 F#m7b5 B7
(The intervals are the same: i7 iiø7 V7.)
Major chords Minor chords Seventh chords Extended chords Sus chords Dim chords Aug Chords Add Chords Altered Chords