Adim is a minor triad with its 5th lowered to : A, C and Eb. That makes two stacked minor thirds.
“Dim” names a three-note triad, not a diminished seventh chord. In a major key, it often appears as a borrowed minor-key ii or a passing chord.
Also written A°, Ao or A diminished.
Adim in 1st inversion puts C in the bass instead of A, the 3rd of the chord. The notes are the same; what changes is what the bass line does, which is most of why you would reach for it — it lets the lowest voice move by step where the root would have jumped.
One of Adim’s 9 shapes does this:x3121x
In pop songs Adim 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 G.
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 Adim: in first inversion, between Bb 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 Adim belongs to G minor and Bb major.
Contains Adim and adds fourth, fifth and flat seventh.
Contains Adim and adds flat second, fourth, sharp fifth and flat seventh.
Contains Adim and adds flat second, major third, sharp fifth and flat seventh.
Contains Adim and adds second, fourth, sharp fifth, sixth and major seventh.
Contains Adim and adds flat second, major third, fifth, sixth and flat seventh.
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.