Fm/C is C, F and Ab. The shape this page opens on is written x33111, lowest string first.

Also written Fm over C or Fm with C in the bass. The same chord either way; which spelling you meet depends on who wrote the chart.
There are 5 common shapes for Fm/C on the guitar.
In which keys. Every note of Fm/C belongs to C minor (the iv), C# major (the iii), Eb major (the ii), F minor (the i), Ab major (the vi) and Bb minor (the v).
The common shapes, written lowest string first: x33111, x33114, 8-8-10-10-9-8, x-11-10-13-13-13, x-11-10-10-13-13.
The common shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.