Monophonic liturgical chant
With the decline of the Roman Empire, the institution destined to perpetuate and expand the musical heritage of antiquity was the Christian Church, but it was not a unified process. Many of the cultural centres of the Western church developed distinctive characteristics while sharing the common heritage of the Hebrew liturgy and Greek culture. In Milan, for example, metrical hymnody, as distinguished from the earlier practice of unmetred psalmody, was cultivated, particularly under the influence of the 4th-century Bishop Ambrose, who first attempted to codify the growing repertory of chants. This body of Milanese church music, therefore, came to be called Ambrosian chant. Somewhat later a unique style and repertory known as Mozarabic chant evolved in Spain, and in France the Gallican style prevailed.
But the mainstream of church music was the type of chant practiced in Rome. Beginning in the late 6th century, according to tradition, with Pope Gregory I, the vast number of traditional melodies that became the foundation for the later development of Western art music were codified and organized. A systematic organization of tonal materials also was gradually accomplished, resulting in the eight church modes. Each melody was assigned a specific function in the services of the liturgical year—some for the mass and some for the divine offices such as Matins, Vespers, and Compline. After a period of assimilation, the Gregorian chant repertory began a process of expansion in the 9th century, when the practice of troping originated. A trope is either a text or a melodic section added to a preexisting melody or a combination of text and music incorporated into existing liturgical music. It is not surprising that church musicians, after years of singing traditional chants, should want to express themselves by adding words to vocalized melodies. Perhaps the motive was more functional: the added syllables would make the long textless passages easier to remember. Tuotilo (died 915), a monk of St. Gall in Switzerland, is credited with the invention of tropes. Notker Balbulus (died 912) is notable for his association with the sequence, a long hymn that originated as a trope added to the final syllable of the Alleluia of the mass.