The modal D tuning for guitar is an alternative tuning for guitar where the six open strings of the guitar (lowest to highest) are tuned to the following notes: D, A, D, G, A, D.
Modal D tuning is similar to the open D tuning (D, A, D, F#, A, D) and the open D minor tuning (D, A, D, F, A, D), but the major and minor third of the D major or minor scale respectively is replaced by the fourth of the D scale (major or minor) (F# or F is replaced by a G). The result – the modal D tuning – is not decisively major or minor. The open strings of this tuning produce the suspended D chord (Dsus4 composed of D, G, and A).
The modal D tuning is interesting also because it allows chords in which different strings can carry the same pitch (same note in the same octave) which produces a droning sound. The open second string (highest to lowest), for example, and the second fret on the third string produce the same A note. In other words, many chords allow these two strings to produce the same pitch with only a two-fret difference. This is easier on the neck hand than standard guitar tuning where the difference is four frets.
The most common guitar tuning is EADGBE (lowest to highest). One can obtain the modal D tuning from the standard tuning by: 1) dropping in the lowest guitar string one tone from E to D; 2) dropping the second string (highest to lowest) one tone from B to A; and 3) dropping the first guitar string one tone from E to D.