guitar chord
F♯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
20421x
hard
2045xx
easy
xx4212
hard
2342xx
moderate · barre
xx4575
hard
What is true of it
F♯dim is F♯, A and C — 1, b3 and b5 counted from F♯.
The easiest of the 5 fingerings below is 20421x: 4 fingers, held at the 1st 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.
1 of the 5 need a barre.
1 string stays silent.
Where its notes are on the neck
The brass dots are F♯, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
F♯dim/A
5045xx
F♯dim/C
x3421x
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.