What is the chant that used as basis for polyphony?

The chant used as the basis for polyphony is Gregorian chant, also called plainsong. This early form of monophonic, liturgical singing was developed in Western Europe during the Middle Ages. Gregorian chant is characterized by its use of plainchant, a style of singing that uses only one note per syllable of text, and its modal system, which is based on eight musical modes. Gregorian chant was the primary musical form used in the Roman Catholic Church for centuries and served as the foundation for the development of polyphony, a musical technique that involves combining two or more independent melodic lines.