About Cdim — triad voicings
Cdim is C, Eb and Gb — Cm with the fifth lowered as well.
It sounds compressed and unstable in every direction. Rarely a destination; it is where a line passes through on the way somewhere.
Cdim notes and formula
| C | Eb | Gb |
|---|---|---|
| 1 | ♭3 | ♭5 |
Unstable — it does not sit still.
Also written C°, Co or C diminished. The same chord either way; which spelling you meet depends on who wrote the chart.
How to play Cdim
Three notes on one string set. No seventh, so nothing is claimed about the chord beyond its quality.
Thin on their own, excellent over a bass line or against someone else’s harmony.
Cdim has 46 of them, on the bottom three strings, the middle four strings, the low-middle three, the high-middle three, the top three strings, the bottom four strings, the top four strings, strings 6-4-3-2 and strings 5-3-2-1, from open position up to the 13th — every inversion the chord has: C in the bass, Eb in the bass and Gb in the bass. The one to learn first is the Gb shape at the nut, which is the easiest grip here.
6 of them sit on the middle four strings, so you can run the inversions up one set without changing strings — which is the whole reason to learn them as a family rather than as separate grips.
Start with x31x12 — down at the nut. None of the shapes for Cdim uses an open string, so every one of them is movable; the lowest sits at the 1st fret. None of them needs a barre. Together they cover the neck from the 1st fret to the 15th.
| Frets | Where | Bass | Top | Hand |
|---|---|---|---|---|
| x31x12 | 1st fret | C, root position | Gb | all fretted |
| x3454x | 3rd fret | C, root position | Eb | all fretted |
| 86x87x | 6th fret | C, root position | Gb | all fretted |
| x-x-10-11-x-11 | 10th fret | C, root position | Eb | all fretted |
| x-15-13-x-13-14 | 13th fret | C, root position | Gb | all fretted |
Tensions. A shell of Cdim, root, third and seventh, leaves a string free, and the tones that belong over it are the 11 (F) and the b13 (Ab).
How to use Cdim
In which keys. Every note of Cdim belongs to C# major (the vii°, the leading tone) and Bb minor (the ii°, the supertonic).
What to play over Cdim
Every note of Cdim is in C blues, C locrian, C altered, C diminished and C half-whole diminished.
Chords near Cdim
Change one note of Cdim and you have Cm (the flat fifth becomes a fifth), Cdim7 (add the sixth), Cm7b5 (add the flat seventh) or CmMaj7b5 (add the major seventh).
Cdim is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished chords on every root.
On Cdim’s sound and use this page follows Stefan Kostka, Dorothy Payne & Byron Almén, Tonal Harmony (McGraw-Hill, 8th ed., 2018). The shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.