How did Claude Debussy challenge traditional rules of harmony?

French composer Claude Debussy challenged traditional harmony by partially abandoning functional tonality, effectively blurring or dissolving the harmonic basis of his works. His innovative approach included several key techniques:12

• Whole-Tone Scales: Debussy utilized the whole-tone scale, which lacks the half steps found in traditional major and minor scales. Because this scale produces only augmented triads, it lacks the standard dissonances that require resolution, allowing his music to exist without traditional harmonic tension and release.132
• Self-Sufficient Dissonance: He treated chords traditionally considered dissonant, such as seventh chords, as self-sufficient harmonies rather than as sounds requiring resolution to a stable chord.1
• Parallel Movement: By using sequences of seventh chords moving in parallel, he obscured traditional harmonic progressions, creating a shimmering effect of "colours" rather than directed motion.41

These methods allowed Debussy to move away from the rhetoric of Romanticism, prioritizing instrumental timbre and atmospheric effect over the established rules of Western musical form.41