Gregorian chant takes its name from Pope St. Gregory the Great. Although the tradition proclaims him as the composer of chant, historical scholarship shows rather that he served as the great link between the early Church and the Middle Ages. As such, he symbolizes the chant of the churches in Rome, that spread to England and to Gaul in the seventh and eighth centuries.
At the behest of Charlemagne (768-814), Frankish musicians in his kingdom were instructed to learn the Roman style of singing at the liturgy. The result of their attempt, neither strictly Roman nor Frankish, is what became known as Gregorian chant. The early development is difficult to trace because all the music was handed on as an oral tradition; nothing was written down even though the repertoire for the Mass and the Divine Office comprised well over 2,000 pieces.
Chant can be divided into three types, marked by the degree of difficulty. Simple chants allowed the whole congregation to participate, and some could easily reach back into antiquity.
More complex are the antiphons for Lauds and Vespers. Still, they are not too difficult for a monastic community with members of varying skills. The "O" antiphons for Advent belong to this second group.
Finally, solo cantors or small groups of trained musicians would sing the complex or melismatic chants for the Propers of the Mass. These complex chants are built upon structural notes that are embellished with elaborate strings of melody, not unlike the Celtic knots found in the art of the Book of Kells. The effect is a kind of medieval jazz.
During the ninth century, a system of notation developed to assist the cantors. Unlike modern notation, which indicates pitch and rhythm, this system of dots and lines sought to preserve the nuances of the oral performance. As time passed, memory faded, and it became necessary to indicate pitch. And so the four-line staff developed with its square notation.
The new system lacked nuance; one grouping of square notes stood for five or more different signs in the old system. Free rhythm and the modal system were also losing ground. The development of polyphony demanded strictly measured time, and the major and minor keys eventually supplanted the modes.
In the mid-nineteenth century, Benedictine monks at the Abbey of Solesmes began collecting and copying the earliest surviving manuscripts in an effort to restore the beauty and complexity of chant that they felt had become obscured over time. Also at issue was early performance practice and interpretation, in particular the question of rhythm. Monk-scholars at Solesmes have proposed several approaches over the course of more than a century of sustained study.