How do consonance and dissonance affect counterpoint?

In counterpoint, the interplay between consonance and dissonance is fundamental to creating musical movement and structure. These elements function as the "vertical" consideration of counterpoint, defining the relationship between simultaneous melodic lines.1

The Role of Consonance and Dissonance

• Consonance: Traditionally includes perfect intervals (unison, octave, fifth) and imperfect consonances (major and minor thirds and sixths). These intervals imply repose and stability.23
• Dissonance: Includes seconds, sevenths, and augmented or diminished intervals (such as the tritone). Dissonance creates tension and instability, which the human ear naturally expects to be resolved through movement to a stable, consonant harmony.243

Application in Counterpoint

Theorists have historically prescribed where these intervals should occur, often restricting dissonances to weak beats of the musical metre and requiring them to resolve via stepwise movement to a consonance. Notably, the perfect fourth is considered a special case; while generally consonant, it is treated as a dissonance in two-part counterpoint when it occurs against the bass. Ultimately, the skillful management of these forces allows composers to balance the independence of individual melodic lines with a coherent, flowing harmonic design.4321