The Fdim7 chord (diminished 7th): notes F G# B D, degrees 1 – ♭3 – ♭5 – ♭♭7. Guitar shapes: 120101, xx3434, x86767, x89797.
A chord is the root plus degrees above it, most often stacked in thirds. Major and minor differ only in the lower third: a major third (M3) sounds bright, a minor third (m3) sounds sad. Pick a chord and a root to see its degrees and intervals, and its shapes.
m — minor, M — major, P — perfect, A — augmented interval; the number is the degree count. m3 is 3 semitones, M3 is 4, P5 is 7
F Fm F5 F7 Fm7 Fmaj7 Fsus2 Fsus4 Fadd9 F6 Fm6 F9 Fdim Fdim7 Fm7♭5 Faug