Mandolin C chordC major: 0 2 3 0, and 5 more shapes

Frets, low course to high: 0 2 3 0 (open position) · C/G

Sounds: G E C E · C is C, E, G (1 · 3 · 5)

Found in: G major (IV), D mixolydian (♭VII), G mixolydian (IV), D dorian (♭VII), G dorian (IV), A dorian (♭III)

Open courses that ring: G and E are notes of C, so they can drone under it.

No 3rd: C5 (5 5 3 3) leaves the 3rd out, so it backs C major, mixolydian, dorian and minor tunes alike.

Tuning GDAE: G D A E, low to high.

The link can’t set the tuning: the app starts on the Irish bouzouki in GDAD. Choose Mandolin once in its Settings and it remembers.

More C shapes

G major (IV, a main chord): G – C – G – D (a plain backing for many reels and jigs)
D mixolydian (♭VII, a main chord): D – C – D (the mixolydian sound: D-C-D)
G mixolydian (IV, a main chord): G – C – G – F (reels in D or A mix)
D dorian (♭VII, a main chord): Dm – C – Dm
G dorian (IV, a main chord): Gm – F – C – Gm (the major IV is the dorian colour)
D minor (♭VII, a main chord): Dm – C – B♭ – C