A prosula is an addition of text to a melismatic medieval liturgical chant, to create a new chant in more-or-less strictly syllabic style. It often incorporates the shorter text of its parent chant completely, but at other times affects only an isolated melisma in a parent chant. It often imitates the parent by assonance in a number of ways; by frequent use, at the ends of phrases, of the vowel on which the melisma was originally sung, or by constructing a new text using all the syllables found in the parent text. The syntactical structure of the prosula text usually follows the musical "syntax" of the parent melody. The earliest layer of prosulae was applied to the chants of the Proper of the Mass, the Alleluias and the Offertories, both melismatic in style, and therefore appropriate candidates for the addition of prosulae.
The manuscripts which preserve the Alleluia prosulae are, in the main, tropers, but Graduals (and sometimes otherwise non-musical manuscripts) also preserve them. Prosulae are concentrated in manuscripts from the Southern German-speaking region, from Northern Italy and Aquitaine, but there are also other "stragglers", including a Beneventan manuscript with a sizeable and largely independent repertory. The earliest sources are from Germany, but many of these contain only few prosulae. The first large repertory of prosulae is found in Aquitanian manuscripts. The Northern Italian group of manuscripts is the latest in date, and contains the greatest number of prosulae. However, the earliest and latest manuscripts preserve a core prosula repertory in what seems to be the original form, whereas disturbances to the "syntactical simultaneity' of a prosula with its parent chant are most frequently encountered in manuscripts from Aquitaine and Benevento. Furthermore, new prosulae composed in these regions are designed in different ways, often not according to "syntactical" criteria.
Whilst, on the one hand, the sources containing the unaltered forms of the prosulae do not seem to have been transmitted by authoritative written exemplars, but rather seem to be individual records of a performance tradition, the Aquitanian and Beneventan sources on the other hand seem to represent a more closed, written transmission. Scribes in these latter localities do not seem to have been in touch with the Italian and Eastern performance tradition: these matters can be discerned through examination of the use of similar sound between parent chant and prosula, the transmission of variants, and rhythmic indications in the manuscripts.
Rhythmic indications found in the German manuscripts suggest that their purpose is on the one hand to accentuate the prosula text rhetorically, and on the other to punctuate the musical syntax of the melody. On the other hand, rhythmic indications found in Aquitanian manuscripts seem to be related to the known rhythm of the parent melodies, without necessarily any clear relationship to the prosula text, which suggests a familiarity with the oral tradition of the chant in general, but not with that of the prosula in particular.
There is some indication from the notation of the Northern Italian manuscripts to suggest that the prosulae were performed there with melodic embellishment. This may have arisen because the pre-Gregorian liturgical chant sung in this locality often consisted of repeated, but varied musical phrases, which might suggest that similar musical repetition and variation occurred in the performance of the prosula, by the alternate performance of portions of the prosula and its parent chant.