How did organum combine a plainchant melody with an organal voice?

In the context of medieval music, organum represents an early form of polyphony that combined a pre-existing plainchant melody—known as the vox principalis or principal voice—with an added vox organalis, or organal voice. The methods for combining these voices evolved significantly over time:12

• Parallel Organum: In the earliest style (9th–11th century), the organal voice simply doubled the plainchant at a fixed interval, typically a perfect fourth or fifth below.123
• Free Organum: Later developments introduced greater melodic independence. The organal voice began to move in oblique or contrary motion relative to the chant, using consonances (unisons, fourths, fifths, and octaves) at phrase beginnings and endings to provide structural stability.23
• Melismatic and Tenor-Based Organum: By the 12th century, the plainchant voice was often slowed down and "held" as a tenor, while the organal voice became a highly florid, melismatic melody sung above it. Eventually, composers like Léonin and Pérotin utilized rhythmic modes to organize these parts, creating complex, multi-voiced textures.12