About the Adim chord
To play A diminished, use the first shape above. Place your index finger on the 2nd fret of the G string, your middle finger on the 3rd fret of the C string, your ring finger on the 3rd fret of the A string, and your pinky finger on the 5th fret of the E string. Strum all four strings and check that each one rings cleanly.
Root
A
Quality
Diminished
Notes
A · C · Eb
How the Adim chord works
A
Root(1)
The home note. It gives the chord its name.
C
Minor third(♭3)
The defining minor interval above the root.
Eb
Diminished fifth(♭5)
A lowered fifth that adds harmonic tension.