Throughout this book, chromaticism has been approached as an elaboration of diatonicism. Chromatic music is based fundamentally on the diatonic major and minor scales, with altered scale degrees and chromatic embellishments resulting from tonicization, modal mixture, chromatic nonchord tones, and other chromatic procedures. Even as tonal function begins to dissolve and the tonal system is stretched to its limits (as described in Chapter 21), tertian sonorities like triads and seventh chords continue to prevail. In efforts to break away from functional tonality completely, composers in the twentieth century explored different ways of using diatonic scales; they also began using entirely new scales, or pitch collections, as source material for their music. These collections of notes, known as the referential collections , include the diatonic, pentatonic, octatonic, whole-tone, and hexatonic collections.