The Prosula
Long before the advent of musical notation, the monks who sang plainchant on a daily basis had begun to embellish at least some melodies both musically and textually. Melismatic chants—the various intonations of the Kyrie and the jubilus of the Alleluia in particular—proved especially inviting for the addition of interpolated texts. In one of the earliest accounts of such a trope, Notker of St. Gall (ca. 840–912, also known as Notker Balbulus, "The Stammerer") explains that the practice arose from a need to make the chants easier to memorize. Notker was a monk who worked at the Benedictine abbey of St. Gall in what is now eastern Switzerland. He eventually collected the prosulae he had written in a Liber hymnorum (Book of Hymns), which he organized according to the cycle of the liturgical year.

"When I was still young, and very long melodies—repeatedly entrusted to memory—escaped from my poor little head, I began to reason with myself how I could bind them fast.

"In the meantime it happened that a certain priest from Jumièges (recently laid waste by the Normans) came to us, bringing with him his antiphonary, in which some verses had been set to sequences; but they were in a very corrupt state. Upon closer inspection I was as bitterly disappointed in them as I had been delighted at first glance.

"Nevertheless, in imitation of them I began to write Laudes Deo concinat orbis universus, qui gratis est redemptus, and further on Coluber adae deceptor. When I took these lines to my teacher Iso, he, commending my industry while taking pity on my lack of experience, praised what was pleasing, and what was not he set about to improve, saying, "The individual motions of the melody should receive separate syllables." Hearing that, I immediately corrected those which fell under ia; those under le or lu, however, I left as too difficult; but later, with practice, I managed it easily—for example in "Dominus in Sina" and "Mater." Instructed in this manner, I soon composed my second piece, Psallat ecclesia mater illibata.

"When I showed these little verses to my teacher Marcellus, he, filled with joy, had them copied as a group on a roll; and he gave out different pieces to different boys to be sung. And when he told me that I should collect them in a book and offer them as a gift to some eminent person, I shrank back in shame, thinking I would never be able to do that."

Source: Notker Balbulus, Preface to Liber hymnorum (Book of Hymns), trans. in Richard Crocker, The Early Medieval Sequence (Berkeley and Los Angeles: University of California Press, 1977), p. 1.