Harmonic Techniques of Debussy

Planing

Harmonic planing is a kind of non-functional triadic harmony.
Harmonic planing moves all voices in parallel motion, thus preserving the same exact voicing in successive chords.
Planing may involve triads, seventh chords (a and b), or other harmonic shapes such as quartal/quintal harmony (c).
Planing may occur in all voices (a) or only some of them (b and c).
Planing should involve at least three voices to be recognizable.
Diatonic planing is when all the chords fit into a diatonic collection. Chord quality will not be preserved throughout the passage; instead, the chord quality will change depending on what is diatonic to the key. This kind of planing tends to sound more cohesive with the surrounding music.
Chromatic planing preserves the exact same chord quality throughout the passage, which necessitates the use of accidentals. A passage of chromatic planing will not fit into a single diatonic key. Chromatic planing has a more striking sound since it is not governed by a larger tonality.

Triadic harmonies are built by stacking thirds; quartal/quintal harmonies are built by stacking fourths or fifths, respectively.
Because fourths and fifths are inversions of each other, quartal and quintal harmonies sound similar (just as harmonies built with thirds and sixths do).
Quartal/quintal harmony fits easily within diatonic and pentatonic collections, both of which are built with fifths.