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