Fdim is a diminished, spelled F, Ab and Cb. In Ukulele (low G) tuning (G3 C4 E4 A4) the easiest shape is x542, which sits at fret 2, spans 3 frets and mutes 1 string. 12 positions for it are shown below, from the nut up to fret 11.
Notes and intervals
Note
F
Ab
Cb
Interval
1P
3m
5d
12 ways to play it
Ordered by how hard they are to fret, easiest first, one per position on the neck. Fret numbers are relative to the position marked beside each grid.
x542sits at fret 2, spans 3 frets and mutes 1 string454xsits at fret 4, needs a barre across 3 strings at fret 4 and mutes 1 stringx878sits at fret 7, spans 1 fret and mutes 1 string10111311sits at fret 10 and spans 3 frets1542sits at fret 1 and spans 4 frets13111311sits at fret 11 and needs a barre across 3 strings at fret 114542sits at fret 2 and spans 3 frets10878sits at fret 7 and spans 3 frets1087xsits at fret 7, spans 3 frets and mutes 1 string1512sits at fret 1 and needs a barre across 3 strings at fret 1108711sits at fret 7 and spans 4 frets1011711sits at fret 7 and spans 4 frets
Keys Fdim belongs to
Fdim is diatonic to 8 of the 96 keys this app knows, playing a different role in each.