D add9 is related to D sus2, so what's the difference? All suspended chords leave out the 3rd note of the scale, in D this is F#. So Dsus2 uses these notes: D, E, A. The sus stands for suspended, the other more common sus chord is a sus4.
The fingering for this inversion involves a small stretch to the 5th fret, with the first finger barring the top 3 strings.
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