At some point in the 7th century, the collected set of liturgical Proper chants was arranged and completed in a way that linked each of them to a precisely defined day of the liturgical year. Recent research points to the probability that this arrangement was made gradually, proceeding from one type of chant to another. Although there existed no master plan to coordinate all the Proper chants of a given day, tradition sanctioned their cohesion. Thus the “properization” was completed, and the result was an admirable structure. Its value was guaranteed on the most fundamental level by the quality of the individual texts, i.e. their biblical origin; on the second level by the theological interpretation defining their approximate liturgical position; and on the third level by their order arising from the arrangement within the annual cycle, the system of anni circulus. This, of course, also applies conversely: the biblical words and their theological-liturgical interpretation have immensely enriched the liturgical year. This arrangement per anni circulum helped to fix the structure of the calendar; it made the individual days more characteristic, well distinguished and memorable. The Roman Fifth Instruction of March 28, 2001 (“Liturgiam authenticam”) frequently refers to the “identity” of the Roman liturgy as something to be preserved. Certainly, the Graduale Romanum arranged per anni circulum is (was?) indeed a preeminent part of this identity!
Before we proceed, two lessons should be drawn from what has beer, said thus far. First, the music of the Proper chants was not an emotional or “feeling”- element of the liturgy. Rather, in its proper way, it has a part to play in the communication system of the liturgy. Second, the congregation did not take part in the performance of the Proper chants, since they pertained to the semi-professional or professional singers. This fact does not at all point: to any lack or deficiency in actuosa participatio, but rather is the manifestation in practice of a basic principle of liturgy: the distribution of roles. The skeleton framework of the rite was presented in the ongoing cantillation and dialogue; the delivery of lections was the task of lectors, the singing of psalms was chiefly the task of the psalmist(s) or the schola.
Laszlo Dobszay, The Bugnini-Liturgy and the Reform of the Reform (2003)