The striking quartal harmonies explored by 20th-century composers such as Bartók and Debussy were by no means new sounds by any reasonable definition. Among many other predecessors, we find that quartal chord formations abound in the works of J.S. Bach and so-called “common practice” tertian harmony; but in this situation we rarely choose to recognize them as such. By labeling a quartal formation as a suspension, we are literally stating that it should be another chord—we actually have the gall to deny that sound its integrity outside of a narrowly-defined relationship to an accepted triadic harmony.
Of course this labeling isn’t done arbitrarily; it’s informed by Bach’s own music, which may waver between modality and tonality at times but is usually grounded in the organizing power of the triad. Given Bach’s own characteristic usage of harmonic suspension (always on a strong beat, resolving on a weak beat), labeling these sonic events as suspensions rather than as distinct quartal harmonies makes sense, and even makes possible some deeper insights—for example, it becomes possible to understand Bach’s frequent placement of suspensions on weak beats in 9/8 and other compound meters as implying a shift of metric pulse; the suggestion of the duple 6/8 in 9/8 (tempus perfectum, the trinity of threes) occurs frequently, and it’s striking just how effective Bach’s cerebral, nerdy obsession with numerology is at portraying deeply-felt human crises of faith. (At least that’s one of the ways his pieces sometimes unfold to me—as musical sermons, of sorts.)
Plainly, thinking of Bach’s quartal sonorities as 4-3 and 7-6 suspensions is both fair and productive. But sometimes our labels seem so obvious that it’s easy to forget the underlying musical context that recommended their adoption in the first place; to this end, I’ve found it useful to fill beginning Music Theory 101 classes with as many different styles and periods as possible, so that the student is able to hear a similar musical element functioning in several different contexts. I have always especially enjoyed directing students to study old blues recordings just after they have acquired facility with dominant seventh chords; most are very surprised that a dominant chord can function as a stable tonic in the context of the 12-bar blues, and a few even flirt with labeling the chord as a V7 of IV, clinging to a musical context that has served them well in the past. Eventually though, most have no problem accepting a dominant quality saddled to a tonic function, and can readily identify other stylistic elements that make this bait-and-switch work. Needless to say, this experience can have enormous ramifications in how one hears (and understands) functional dominants in baroque music, and one former student has told me the exercise helped her to more easily hear the augmented sixth chord as a kind of predominant chord, despite its dominant quality. Hearing that familiar dominant chord in a new context reminded her not to take its function in the old context for granted, either.
That’s the reason I want those beginning theory classes to spend time with Bartók, Debussy, and even less textbook-friendly sources too, and not just the “common practice” harmony that isn’t even an acceptable stand-in for the 150 years or so of Western music it supposedly dilutes. When musicians are trained in only one stylistic context, they will be less able to assimilate foreign experiences and importantly, less able in their chosen context, too. Is it any wonder that when most educational programs require music students to “master” conventional tonality before exploring the music of the 20th century and after, most have a hard time hearing other sources of music—and sadly, the music of our own time—as anything other than strange, foreign, or “wrong”?