Cdim is C, Eb and Gb — 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.
Unstable — it does not sit still.
Also written C°, Co or C diminished.
In pop songs Cdim nearly always sits on the second degree, borrowed from the minor key. Its fifth is the key’s flat sixth, and it falls a half step back to the fifth of Bb.
Listen for: The fifth of the I rises a half step and falls back. Every other voice is a neighbour too.
Heard in“Cara Mia”“The Air That I Breathe”“Sleep Walk” (going on to V)
The commonest classical use of Cdim: in first inversion, between Db and the same chord with its third in the bass. It stretches the home chord while the bass climbs by step.
Listen for: The bass climbs a whole step twice, root to third of the I, while the notes above step down and back.
In which keys. Every note of Cdim belongs to C# major and Bb minor.
Every note of Cdim is in C blues, C locrian, C altered, C diminished and C half-whole diminished.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Cdim sits in it. Every scale.
Change one note of Cdim and you have (the flat fifth becomes the fifth), (add the diminished seventh), (add the flat seventh) or (add the major seventh).
Cdim is a diminished chord — root, minor third, flat fifth. Diminished and half-diminished chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.