Double D is what double C becomes when you want to be in D without a capo. It moves the most strings of any tuning here, which is the price of getting a D-rooted open set with the drone raised to match.
The retune, string by string
Derived from pitch arithmetic against standard gDGBD — every octave here is computed as the shortest move that produces the named letter, not looked up:
| String | Standard | Double D | Semitones |
|---|---|---|---|
| 5 (drone) | G4 | A4 | up 2 |
| 4 | D3 | D3 | stays |
| 3 | G3 | A3 | up 2 |
| 2 | B3 | D4 | up 3 |
| 1 | D4 | E4 | up 2 |
aDADE — 4 strings move (5, 3, 2, 1).
What the open strings spell
The distinct letters sounding are D, A, E, which reads as Dsus2 or Asus4. Standard tuning, for comparison, spells G major. Which of those readings is the tonic is a fact about the tune you are playing, not about the strings — so this page names every reading that fits rather than picking one and calling it the answer.
What survives
The standard chord library does NOT apply here — a neck string moved, so every fret in it is off by that string's delta. Concretely: string 5 moves up 2, so anything you fretted there is now 2 frets lower; string 3 moves up 2, so anything you fretted there is now 2 frets lower; string 2 moves up 3, so anything you fretted there is now 3 frets lower; string 1 moves up 2, so anything you fretted there is now 2 frets lower. That is not a reason to avoid the tuning; it is a reason to learn it as its own instrument rather than as a transposition of the one you know.
Get there with the tuner, check the open strings against each other by ear, and then use the fretboard explorer on the standard neck to see how far the map has moved. The capo pages cover the other way to change key — the one that leaves every shape where it was.