Fdim is a minor triad with its 5th lowered to : F, Ab and Cb. That makes two stacked minor thirds.
“Dim” names a three-note triad, not a diminished seventh chord. In a major key, it often appears as a borrowed minor-key ii or a passing chord.
Also written F°, Fo or F diminished.
In pop songs Fdim nearly always sits on the second degree, borrowed from the minor key. Its fifth is the key’s flat sixth, and it falls a half step back to the fifth of Eb.
Listen for: The fifth of the I rises a half step and falls back. Every other voice is a neighbour too.
Heard in“Cara Mia”“The Air That I Breathe”“Sleep Walk” (going on to V)
The commonest classical use of Fdim: in first inversion, between Gb and the same chord with its third in the bass. It stretches the home chord while the bass climbs by step.
Listen for: The bass climbs a whole step twice, root to third of the I, while the notes above step down and back.
In which keys. Every note of Fdim belongs to Eb minor and F# major.
Contains Fdim and adds fourth, fifth and flat seventh.
Contains Fdim and adds flat second, fourth, sharp fifth and flat seventh.
Contains Fdim and adds flat second, major third, sharp fifth and flat seventh.
Contains Fdim and adds second, fourth, sharp fifth, sixth and major seventh.
Contains Fdim and adds flat second, major third, fifth, sixth and flat seventh.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.