How did the tenor voice develop from the chant voice during the 12th century?

During the 12th century, the role of the chant voice underwent a significant transformation as Western music transitioned into true polyphony. Previously, in early organum, the chant voice (or vox principalis) served as the primary melody, often doubled by an organal voice in parallel motion.12

As polyphonic styles evolved, particularly within the Notre-Dame school, a new hierarchy emerged. The chant voice was relegated to the lower part of the composition, where its notes were significantly prolonged or "held" to provide a foundation for more florid, individualized melodies sung above it. Because of this function, the part became known as the tenor, derived from the Latin tenere, meaning "to hold".13

This development allowed the upper voices to move with greater rhythmic independence and complexity, effectively establishing the tenor as the structural pillar upon which the contrapuntal texture was built.12