The Cm chord, and guitar shape chartThe C Minor chord (also written Cmin or Cm) is made up of the notes C (root), E Flat (third) and G (fifth), a structure common to all major and minor chords. While it is quite straightforward on the piano (substitute E Flat for the E Natural of the chord of C Major), on the guitar it can be very awkward for the player who is limited to open chords.
The problem is that the top and bottom strings of the guitar are normally both tuned to the note E (two octaves apart, of course), a note which cannot normally be used with the Cm guitar chord, because that is the note which gives C its major quality. For these strings to be useful in any Cm, they both would have to be fretted at the third fret or higher, which on the guitar makes a C Minor bar chord a much more realistic proposition - unless you have six fingers on your fretting hand!
Open guitar chords of C Minor are possible, but they involve making sure that at least these two outer strings are not sounded, or the resulting chord will be a weird mixture of major and minor. So Cm is not really one for a beginner on the guitar.
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These diagrams show various forms of Cmin for standard string tuning.
And derived from CmSome more music theory: adding a B Flat to the existing notes C, E Flat and G would turn the chord into C Minor Seven (or Cm7 for short). Relatively straightforward for the pianist, but on the guitar it has to be a bar chord. For both of the bar chords given above, just remove finger 4. This gives the shape of Em7 at the 8th fret (fourth diagram), which with only one fretting finger used must be the easiest barred chord to play.