In music, harmony built on the intervals of fourths. Traditional Western music uses tertian harmony - chords are built on thirds, major and minor. Paul Hindemith, et al, sometimes used fourths - 20th Century music is often about breaking with Tradition, after all. In jazz, it was Bill Evans' strategy in dealing with the new terrain of Kind of Blue; McCoy Tyner later turned it into a full-blown (and much-imitated) aesthetic.