Contributed to the music theory of early Christian chant.

Pope Gregory with the Gregorian Chant

Explanation:

The Gregorian Chant it is a genre of monophonic, monodic vocal music (just a melody), not accompanied, or accompanied only by the repetition of the main voice with the organ, with the free and unmeasured rhythm, used by the ritual of the Roman Catholic liturgy. Around the 6th century, Pope Gregory selected, compiled and systematized ecclesiastical songs and different Western liturgies throughout Europe with the aim of unifying them for use in the religious celebrations of the Catholic Church. It is from its name that the term Gregorian derives.

Although popular legend credits Pope Gregory I with inventing Gregorian chant, scholars believe that it arose from a later Carolingian synthesis of Roman chant and Gallican chant. Multi-voice elaborations of Gregorian chant, known as organum, were an early stage in the development of Western polyphony.