Voicings

D9sus4

Fourths with a 3rd on top

Drop 2QuartalUpper structureQuartal
D3 G3 C4 E4Loading piano…
  1. D3R
  2. G34
  3. C4b7
  4. E49

Guide tones carry the chord's sound: the 3rd (4th on sus chords) and the 7th (or 6th).

Source: Common practice