The structure of the Roman Office remained untouched in every important aspect from the 4th or 5th until the beginning of the 20th century. The repertory grew, the number of antiphons and responsories became multiplied during the centuries: If we count with an old repertory that contained the per annum antiphons and the responsories taken from the Psalter, the psalmic pieces of some great solemnities and the common of saints, then the size can be estimated to be no more than 4—5 hundred antiphons and 2-3 hundred responsories in St Benedict’s time. In contrast, the content of an average medieval antiphonary includes two thousand antiphons and one thousand responsories or more. New and new items, later whole cycles (“historiae”) were composed to embellish the new feasts, to solemnize the cult of saints celebrated only by a common Office earlier. The more ancient a piece is, the more it figures as part of the common Roman heritage of Europe. The younger items appear as local additions to the basic antiphonary, always inspired of course by the age old tradition. Both layers have their own value, though their prestige is of different rank.
Laszlo Dobszay, The Bugnini-Liturgy and the Reform of the Reform (2003)