Debussy and counterpoint: the statement initially sounds preposterous. Debussy is normally remembered as the quintessential harmonist, who flaunted the conventional rules of voice leading. When quizzed by his teacher Ernest Guiraud in 1889 or 1890, he famously denied that there was any need to resolve French sixth chords or any reason to avoid parallel triads. According to him: “pleasure is the law.” And when he addressed the same issues later in his career, Debussy went so far as to denounce the conventional distinction between so-called perfect and imperfect chords: “Nothing is more mysterious than a perfect chord! Despite all theories, both old and new, we are still not sure, first why it is perfect, and second, why the other chords have to bear the stigma of being imperfect. Music ought therefore to free itself as quickly as possible from these little rituals with which the conservatories insist on encumbering it.”

When Debussy denounced traditional rules of counterpoint, he challenged an area of music theory whose history dates back many centuries. Vast in scope, contrapuntal theory deals with the conditions under which voices can be stacked above or below one another. Traditional species counterpoint focuses on three main issues: how individual voices proceed from one note to the next (e.g., the predominance of steps over leaps); how stacked voices proceed in relation to one another (e.g., the role of oblique, contrary, similar, or parallel motion); and how far apart those voices should be stacked (e.g., the classification and treatment of consonances and dissonances). Learned counterpoint then considers the ways in which specific melodic lines can be transformed (e.g., transposition, inversion, retrograde, retrograde inversion, augmentation and diminution), restacked (e.g., invertible or double counterpoint at the octave, tenth, and twelfth), and staggered (e.g., strict canonic imitation, free fugal imitation, and stretto).

Although contrapuntal theory was originally developed to explain modal polyphony from the Middle Ages and Renaissance, it was subsequently adapted to cover tonal works from the common-practice period, including the scholastic fugue. These adaptations were necessary because the individual voices now operated within the context of functional harmony. In styles where the textures are controlled harmonically rather than intervallically, the individual lines can contain more frequent and more extreme leaps; in so-called compound melodies such leaps are created by shifting from one chord tone to another or from one “latent voice” to another.