How did ideas about consonance and dissonance change across music history?

The perception and regulation of consonance and dissonance have evolved significantly throughout Western music history, shifting from strict mathematical definitions to expressive tools for harmonic color.12

Early Concepts and Regulation

In the medieval period, music theory prioritized "perfect" consonances—intervals with simple frequency ratios like the octave (1:2), fifth, and fourth. Dissonant intervals, such as the second and seventh, were restricted to weak beats and required resolution to a consonance. The tritone, or augmented fourth, was famously avoided as an intolerable dissonance.34

The Common Practice Period

By the 18th century, the focus shifted to the triad as the foundational harmonic unit. Functional harmony emerged, where dissonance—such as the dominant seventh chord—was used to create tension that pulled toward a stable tonic chord. Composers increasingly used suspensions to delay resolution, heightening the expressive potential of harmonic movement.35

Romanticism and Beyond

In the 19th century, composers began using dissonance for its own sake—as "harmonic color"—rather than solely for functional resolution. Works like Wagner’s Tristan und Isolde pushed these boundaries, using chromaticism and ambiguous chords to blur tonality and express complex emotions like restlessness or unrequited passion. By the late 19th century, the traditional diatonic system began to dissolve, leading composers to develop highly individual, experimental uses of dissonance that moved beyond the strict rules of earlier eras.6