guitar chord
A♭dim
Two minor thirds stacked. The fifth is diminished, so it cannot be dropped without losing the chord.
Written the other way round it is G♯dim — the same twelve keys on a piano, a different piece of notation.
Every fingering that fits a hand
450404
moderate
420404
moderate
45040x
easy
42040x
easy
420104
moderate
What is true of it
A♭dim is A♭, C♭ and E♭♭ — 1, b3 and b5 counted from A♭.
E♭♭ is a double flat, and it has to be: every letter from A♭ upwards is used once, so the b5 cannot be written any other way.
The easiest of the 5 fingerings below is 450404: 4 fingers, held at the 4th fret.
None of them uses an open string, so this chord moves up the neck unchanged — the same shape 1 fret higher is the chord a semitone above.
Where its notes are on the neck
The brass dots are A♭, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
A♭dim/C♭
x2010x
A♭dim/E♭♭
xx0404
An inversion is the same chord with a different note lowest. The name changes to a slash name because the bass is what a listener hears as the foundation.