Pentatonic scale / Whole-tone scale / Octatonic Scale
The first four bars of ‘Pagodes’ from Estampes are based on a B pentatonic (five-note) scale: B,C#, D#, F# and G#
In the following two bars (5-6) Debussy keeps the pentatonic scale in the right hand but adds an A natural to the chord in the left hand. This is rather like Western Classical Tradition composers adding chromaticisms to major or minor keys to vary the colour and keep the music moving.
Debussy sticks with the same G pentatonic scale (G, A, B, D, E) for longer in ‘The Sunken Cathedral’ from Preludes, creating a rather static portrayal of a cathedral under the ocean. Debussy is interested here in colour, texture and atmosphere. He builds up chains of open fifths over a drone, creating a wash of sound with the sustaining pedal.