Voicings

C9sus4

Bb triad over C (sus)

Drop 3Upper structureUpper structures
C3 Bb3 D4 F4Loading piano…
  1. C3R
  2. Bb3b7
  3. D49
  4. F44

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

Source: Common practice