The important contribution of Heinrich Isaac (ca. 1455–1517) to the genre of the proper of the mass has long been recognised. His work in this genre, collected in the monumental posthumously published Choralis Constantinus, was considered a landmark even in the sixteenth century. Yet Isaac’s magnum opus was by no means isolated. The mass proper played a much greater and more significant musical and symbolic role in the landscape of later-medieval and Renaissance music-making than is currently acknowledged. The present collection of fifteen essays offers new insights into both Isaac's mass propers themselves, which are still shrouded by many enigmas, and their context within broader later-fifteenth and sixteenth-century mass proper traditions. The circumstances under which Isaac's mass propers were composed, performed, and transmitted are discussed afresh, as is the striking late-sixteenth-century reception that the Choralis experienced. Studies of previously unknown or little-examined mass proper collections from countries as widely seperated as Portugal and Poland, as well as of the transformation of the genre in Lutheran territories and in the hands of William Byrd, show that Isaac's enterprise, though the largest of its kind, was built on and embedded in a strong and ongoing tradition of proper settings and cycles.

Ms. 967 in the Arquivo Distrital of Braga, Portugal, is a choirbook, compiled around 1550, containing forty introit/communion polyphonic pairs for the Proper of the Time, from Advent to the tenth Sunday after Pentecost, and five more pieces. The texture varies from four to six parts, depending on the liturgical importance of the feast. The cantus firmus is written in black square notes and twice named Tenor in cantu planu, while the counterpoint appears in round white mensural notation. The selection of texts is in accordance with the Braga Missal of 1498. Of the eighty-five pieces, thirty-three are explicitly attributed to [Miguel da] Fonseca, choirmaster of Braga cathedral around 1544. Most of the remaining pieces are probably by him as well, on account of musical style and external concordances (MM 40 in the Oporto Public Library, copied ca. 1590). In this paper this unknown source is described and illustrated and the possible relationship with the counterpoint rules expounded by Mateus de Aranda (Tractado de canto mensurable y contrapuncto, Lisboa, 1535) is explored.

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   |texte=   The Liber Introitus of Miguel da Fonseca, and a Possible Improvisatory Model
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