Mandolin D chordD major: 2 0 0 2, and 5 more shapes

Frets, low course to high: 2 0 0 2 (open position) · D/A

Sounds: A D A F♯ · D is D, F♯, A (1 · 3 · 5)

Found in: D major (I), G major (V), A major (IV), D mixolydian (I), A mixolydian (IV), E mixolydian (♭VII)

Open courses that ring: D and A are notes of D, so they can drone under it.

No 3rd: D5 (2 0 0 x) leaves the 3rd out, so it backs D 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 D shapes

D major (I, a main chord): D – G – D – A (a plain backing for many reels and jigs)
G major (V, a main chord): G – C – G – D (a plain backing for many reels and jigs)
A major (IV, a main chord): A – D – A – E (a plain backing for many reels and jigs)
D mixolydian (I, a main chord): D – C – D (the mixolydian sound: D-C-D)
A mixolydian (IV, a main chord): A – D – A – G (reels in D or A mix)
E mixolydian (♭VII, a main chord): E – D – E