Why did Gregorian chant serve as source material for medieval and Renaissance polyphony?

Gregorian chant served as essential source material for medieval and Renaissance polyphony primarily because it provided a foundational structure known as the cantus firmus. In this technique, a pre-existing plainchant melody was used as the structural basis for a polyphonic composition, with other voices added above or below it.1

Beyond its functional role as a musical anchor, the use of Gregorian chant held significant symbolic importance within the Roman Catholic Church, as it formed the core of the liturgical repertoire. The practice of building upon these chants allowed composers to integrate traditional, sacred melodies into increasingly complex polyphonic forms, such as the motet and the mass.123

Furthermore, the modal nature of Renaissance counterpoint shared a close kinship with the traditional modality of liturgical plainchant, making it a natural and authoritative choice for sacred musical settings. By utilizing these established melodies, composers maintained a connection to the historical European ideal of composition while developing the sophisticated, imitative textures that defined the era.3