Musicae Sacrae §14

¹⁴ The choral chant began to be called "Gregorian" after St. Gregory, the man who revived it. It attained new beauty in almost all parts of Christian Europe after the 8th or 9th century because of its accompaniment by a new musical instrument called the "organ." Little by little, beginning in the 9th century, polyphonic singing was added to this choral chant. The study and use of polyphonic singing were developed more and more during the centuries that followed and were raised to a marvelous perfection under the guidance of magnificent composers during the 15th and 16th centuries.