Fdim is a diminished, spelled F, Ab and Cb. In Drop D tuning (D2 A2 D3 G3 B3 E4) the easiest shape is 32310x, which sits at fret 1, spans 2 frets, lets 1 string ring open and mutes 1 string. 12 positions for it are shown below, from the nut up to fret 13.
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.
32310xsits at fret 1, spans 2 frets, lets 1 string ring open and mutes 1 stringxx3404sits at fret 3, spans 1 fret, lets 1 string ring open and mutes 2 stringsx23404sits at fret 2, spans 2 frets, lets 1 string ring open and mutes 1 stringx89109xsits at fret 8, spans 2 frets and mutes 2 stringsxx6464sits at fret 4, needs a barre across 3 strings at fret 4 and mutes 2 strings151415130xsits at fret 13, spans 2 frets, lets 1 string ring open and mutes 1 stringxxx1097sits at fret 7, spans 3 frets and mutes 3 strings9119100xsits at fret 9, needs a barre across 3 strings at fret 9, lets 1 string ring open and mutes 1 stringx861007sits at fret 6, spans 4 frets, lets 1 string ring open and mutes 1 stringxxx131213sits at fret 12, spans 1 fret and mutes 3 strings151115130xsits at fret 11, spans 4 frets, lets 1 string ring open and mutes 1 string3231xxsits at fret 1, spans 2 frets and mutes 2 strings
Keys Fdim belongs to
Fdim is diatonic to 8 of the 96 keys this app knows, playing a different role in each.