F Diminished

Fdim is a solid intermediate shape once open chords feel steady. The first shape is the easiest voicing: 4 fretted strings and 0 open strings. Put your index finger on the 1st fret of the G string. Put your ring finger on the 2nd fret of the C string. Put your middle finger on the 1st fret of the E string. Put your pinky finger on the 2nd fret of the A string. Strum all four strings and check each one rings cleanly.

Fdim is F, G#, B. That is a diminished triad (root, minor third, diminished fifth). Play them in any order and you still get Fdim.

Tense and unstable: a passing chord that pulls strongly toward resolution. Chord quality is the interval pattern above the root. That pattern is what you hear as diminished.