The idea
Quartal voicings
Instead of stacking the chord in thirds, you stack it in fourths. The voicing loses its obvious major-or-minor face and opens into something wide, modern and slightly ambiguous — the sound of McCoy Tyner's left hand, and of a guitar that suddenly seems larger than six strings.
Try it now
Play a ii–V–I in C. On each chord, play only this three-note stack of fourths, moving it to the nearest chord tone rather than sliding it in parallel. Listen for the moment it stops sounding like a mistake and starts sounding like a colour. Two minutes, no metronome.
G–C–F — two stacked perfect fourths
Go deeper
74 studies build this idea.
22 of them are free — the open strings of this collection. Start there; a wrong guess costs you nothing.