How did organum create the first forms of medieval polyphony?

Organum represents the earliest form of polyphony in Western music, emerging during the Middle Ages as a technique for adding a second voice to Gregorian chant.12

The development of organum progressed through several stages:

• Parallel Organum: The earliest documented form, described in the 9th-century treatise Musica enchiriadis, involved adding a second voice (vox organalis) that moved in parallel motion at the interval of a fourth or fifth below the original chant (vox principalis).123
• Free Organum: Composers began incorporating contrary and similar motion, allowing the added voice to gain melodic independence. This style utilized consonances—specifically fourths, fifths, and octaves—at phrase beginnings and endings to create a sense of resolution, establishing early concepts of harmonic tension and cadence.23
• Melismatic Organum: By the 12th century, particularly at the Notre-Dame school, the chant was relegated to sustained notes in the lower voice (the tenor), while an upper voice performed elaborate, florid melodies. This evolution culminated in the complex, rhythmically structured compositions found in the Magnus liber organi.142