Open C tuning
The open C tuning is an alternative tuning for guitar. The tunings could for exemple be used in folk style or in slow blues.
To get the open C tuning on your guitar you tune it like this: CGCGCE. The strings that differ from the standard should be turned so the string tension is decreased and not the opposite. The exception is the second string which should be tuned up from B to C. This will increase the tension on that string, but only with a semi-step which normally don't result in string breakage.
Including an octave perspective, the tuning is written C2-G2-C3-G3-C4-E4, meaning that the lowest string is a C note on the second octave, the second lowest string is a G note on the second octave and so on. Standard tuning reference: E2-A2-D3-G3-B3-E4.
There are obviously plenty of possible chord shapes for C major chords. Dm chords are more problematic, if you want to play the Dm triad one possibility are XX2221.
The Em chord version has an omitted fifth and is more correctly written Em(no5), but is chosen here since it's a straightforward way to play the chord. An alternative that include the fifth is XX4443.
The F and G chords could also be played half-barres as XX5575 and XX7777, respectively; or in open position as 020201 (F/C) and X02023 (G5).
The get a more distinctive A minor 7th sound, X20045 is an option. The ninth is omitted in the Am11 version.
It's quite hard to find convenient shapes for many of the most common diatonic key chords in this tuning, but on the same time there are lots of possibilities for voicings.
This a one simply approach to play voicings by using identical or similar shapes with a clear pattern. You could for example alternate the shapes by involving a third finger on the third string. For example, would 570700 result in a Dm9 chord.
There are tons of possibilities for beautiful voicings with open ringing strings suitable for the key of C major and minor in this tuning, also playing in G key works fine. If you want to go more into depth of this particular guitar tuning, see the Essential Chords in Open C Tuning ebook with over 100 chord diagrams.
Additional open chords in this tuning:
See above for pdf chord chart ("Essential Chords in Open C Tuning ebook") for many more voicings, progressions etc.
In the C minor key
This tuning can also function for C minor, especially using chord voicings which avoid involve the high E string as open.
Some examples of chord voicings:
Bbadd9: 000 10 10 10
Eb6/G: 00012 12 11
Alternative Open C tunings
Besides the CGCGCE version presented here, there are some “alternative C Open tunings”. The alternatives are foremost CGCGGE, CGCEGC and EGCGCE (all alternative C Major configurations). In addition, there are the Open C6 Tuning (CGCGAE), Open Cmaj7 Tuning (CGEGBE) and the Csus2 Tuning (CGCGCD).