“Hic maxime Ecclesiasticum ornavit cantum”

What was expected of a “true composer”? Isaac’s contract with Maximilian makes it clear that the emperor wanted music for his chapel, where he hoped to draw together outstanding musical specialists with various skills. Although the contract gives no details about particular genres or works to be composed, insight can be gained by looking at the sacred music that Isaac produced once he entered Maximilian’s service. In particular, he cultivated to an unprecedented degree two regionally specific musical forms almost entirely absent in his earlier work and in the outputs of his contemporaries: the chant-based alternation (alternatim) mass ordinary cycle, and the polyphonic mass proper cycle.

The alternatim ordinary cycle takes the relevant plainchant as its basis in each movement: the Kyrie takes a Kyrie chant, the Gloria a Gloria chant, and so forth. This practice harks back to the earliest polyphonic mass ordinary settings, but had been overshadowed, from the early fifteenth century onwards, by cyclic techniques in which the same borrowed material was cited in each movement. Isaac composed twenty-one alternatim cycles, systematically exploring scorings from three to six voices, and festal types from the most everyday, to the most important, including Easter and solemn and Marian feasts. Sections treating parts of the plainchant in polyphony are typically relatively short, and alternate with either improvised organ music or monophonic chant.[17]

The mass proper refers to those parts of the mass with texts and melodies that change from feast to feast. The practice of setting these in polyphony is traceable to the earliest surviving sources for polyphony of any kind: the first major repertory of practical polyphony, contained in the so-called Winchester Troper (c. 1020–30), consists largely of such settings;[18] so too does the first polyphonic repertory with fixed rhythm, associated with Notre-Dame cathedral in Paris in the late twelfth and early thirteenth centuries.[19] After these spectacular early witnesses, however, the further development of the genre is difficult to trace as it is only poorly represented in sources from the subsequent two centuries. Substantial collections of polyphonic propers appear again from the mid-fifteenth century, the most important ones being connected in some way to the imperial court.[20] These sources suggest that the court regularly sang the introit and sequence polyphonically by the time of Isaac’s appointment, and form the backdrop to his monumental and unparalleled contribution to the genre.

Some 400 of Isaac’s mass proper settings survive, grouped into cycles for feasts throughout the year.[21] Isaac set the introit, alleluia, sequence (if appropriate), and communion. As with the alternatim ordinaries, the settings are based on the relevant plainchant melodies (» Hörbsp. ♫ Puer natus est nobis, Isaac).[22] Isaac’s overwhelming cultivation of the genre in service of the Habsburg chapel inspired envy and long-lasting admiration: early in the production of the imperial series, the court of Elector Friedrich the Wise copied a significant number of the cycles;[23] and in 1507–1508, the cathedral at Konstanz succeeded in recruiting Isaac himself to compose a series of mass propers for high feasts for them.

Maximilian clearly sought to monumentalize the liturgy of his chapel with appropriate, chant-based polyphonic music for every occasion. Isaac’s alternatim ordinaries and mass propers were as striking to his contemporaries as they still are to us in this respect. In his summary of Isaac’s achievements, Heinrich Glarean, one of the most important music theorists of the sixteenth century, concluded that, “He embellished plainchant especially; namely he had seen a majesty and natural strength in it which surpasses by far the themes invented in our time.” [24] The status of Isaac’s chant-based music can be judged from the publication, in the mid-sixteenth century, of a three-volume collected edition of his mass propers, under the title » Coralis constantinus (vol. 1: 1550; vols. 2 and 3: 1555; » Abb. Title page Coralis constantinus).[25] At the time, such a single-author edition, produced so long after a composer’s death, was unprecedented. Not even Josquin received such lavish posthumous attention. The print provided a core mass proper repertory for numerous institutions throughout the second half of the sixteenth century.[26]

Abb. Title page Coralis constantinus

Title page of the tenor part book of Isaac’s » Primus tomus Coralis Constantini  (Nürnberg: Hieronymus Formschneider 1550).