D Major
0 - 4 - 7Root + major 3rd + perfect 5th
About D Major
D major is the chord the guitar was almost built for. Three fingers form a triangle on the top three strings, the open D underneath gives the root, and the whole thing rings without effort. Its key has two sharps — F♯ and C♯ — and both fall where the hand already is.
The notes
D, F♯, A — root, major third, perfect fifth. The open D string supplies the root and the open A supplies the fifth, so only three notes need fretting, and the highest of them, F♯, is the note that makes the chord major.
Lower that F♯ by a fret and you get D minor; leave it out and add E instead and you get Dsus2, one of the most-used chords in fingerstyle. The whole family lives within one finger of the same shape.
Why D is a guitar key
Drop the low E to D and the instrument tunes itself to this chord — that is what drop-D tuning is, and it exists because D major is already where the guitar resonates. Even in standard tuning, the open D and A strings ring underneath the fretted notes and thicken the sound at no cost.
The chords of the key — D, Em, F♯m, G, A, Bm — include only one that requires a barre. Compare that with any flat key, where most of them do.
Where it shows up
As tonic of its own key, as the IV of A major and the V of G major. That last role is the most common in guitar music: G–C–D and D–A–G are the backbone of folk, country and early rock alike.
Add the little finger at the third fret of the high E and D becomes Dsus4, whose release back to plain D is one of the most recognisable gestures on the instrument.