How did organum become the earliest form of Western harmony?

Organum emerged in the 9th century as the earliest form of Western harmony, originating from the practice of adding a second, harmonizing voice to Gregorian chant melodies. Initially, this technique—documented in the Musica enchiriadis—involved "parallel organum," where a second voice moved in strict parallel motion at the interval of a fourth or fifth below the principal melody.123

Over time, this practice evolved into "free organum," where the added voice gained melodic independence, often moving in contrary motion to the original chant. This development introduced a fundamental concept of Western harmony: the movement from tension (dissonance) to repose (consonance), particularly at phrase endings.23

By the 12th century, the Notre-Dame school in Paris refined these techniques into elaborate, multi-part compositions. In this "melismatic" style, the original chant was sustained in long notes while upper voices sang florid, rhythmically independent melodies above it, marking a significant step toward the complex polyphonic traditions that would define Western music for centuries.143