An alternatim setting of four Mass Ordinary movements to be performed in the Easter season, this is the grandest of no less than four Paschal masses from Isaac's pen. The work is scored on a monumental scale, with six independent voice parts expanding on occasion to execute final sonorities of up to ten notes (as at the end of the Kyrie, where the "Discantus secundus" splits into four parts). Contrasting notably with the mass's rich and unusually full texture (even the Criste settings here retain the full six voices), the temporal extent of the individual polyphonic sections remains remarkably limited. The longest section in the mass, the Qui tollis, lasts a mere 42 measures, and the opening item of the Gloria (Laudamus te) is allotted no more than eight measures of tempus imperfectum (enough time for the Discantus to produce an undecorated version of the chant in equal breves, followed by a brief ending flourish in the first Bassus). In this aspect of the setting, Isaac stays in line with typical alternatim practice - since the polyphonic segments set only every other text phrase, they can never approach the extent of sections in larger-scale cyclic masses - but the large-scale scoring sets this mass on a different footing than the typical examples of such functional liturgical polyphony.¹ The resulting aural impression is of a remarkably lucid and straightforward elaboration of pre-existent materials, a seemingly effortless creation which never taxes the listener despite the complexities of its six-part scoring.

A significant model for Isaac's treatment of large-scale sonorous writing based on a "structural" cantus firmus must have been the tenor-motets of Johannes Regis.² If the predilection for expanded final sonorities mentioned above offers only a vague hint of Regis's influence (a characteristic which was widespread by the end of the 15th century), the structuring of a section like the superb second polyphonic Criste (Kyrie mm. 64-92) recalls the earlier master's practices more explicitly. Particularly the strictness and the frequency of the alternation between full scoring in slow note values, on the one hand, and extremely brief but more florid duets, on the other, offer noteworthy parallels to the secunda pars of Regis's celebrated Clangat plebs, and less strictly the secunda pars of Lux solempnis adest. A demonstration of Isaac's concern with the structuring force of such a layout is offered by the numerical balance in this Criste. The seven-part structural division Full-Duet-Full-Duet-Full-Duet-Full is matched to a temporal partitioning based equally on the number seven, with exactly seven measures of duets being matched to twenty-one measures of full scoring:³

Table 1: Seven-part layout of Criste III (Kyrie mm. 64-92)

As in Requiem settings and other polyphonic items which follow the appropriate plainchants of each section/movement closely (as opposed to cyclic masses in which each movement is based on the same thematic material), the tonal character of this mass undergoes shifts from movement to movement. The "Phrygian" ending of the first Kyrie sits in the context of a movement centered mostly around G, although a characteristic harmonic move for Isaac is the temporary introduction of F-based segments within this different overall tonality, as in the Criste settings, mm. 43-9 and 75-81. This F tonality returns after the G-based Gloria, in the Sanctus and Agnus dei (which now include round b signature clefs in each voice), so the composition observes on a large scale a division into two sections (G-based Kyrie and Gloria, F-based Sanctus and Agnus dei; no Credo is set by Isaac) mirroring the pairings found in the chant sources.

This disjunction between the two halves of the mass is reflected further in its transmission patterns, in which the different movements seem to have become quite easily separated from each other. Of the three German sources (BerlPS 40013, JenaU 36, and NurGN 83795), two transmit Kyrie/Gloria and Sanctus/Agnus as pairs with other music in between. The Alamire book VatS 160 preserves only the Kyrie, and the Occo Codex places two four-voice Paschal Kyrie settings in between the Kyrie and Gloria, apparently offering them as alternative settings within Isaac's mass, to judge by the title on f. 117v: "Missa paschale sex vocum . henricus ysaac. et cetera." JenaU 36 is thus the only source to transmit the mass as a single uninterrupted unit.

Alamire's version of the Sanctus within the Occo Codex has also apparently modified the sectional structure of the composition to reduce the amount of alternating plainchant, by a simple act of re-texting which leaves Isaac's polyphony entirely intact. The Pleni sunt and Benedictus here end with the text "Osanna in excelsis" in all voices, contradicting both the texting of the paraphrased plainchant melodies (which stop before Osanna in the segments quoted by Isaac) and the texting presented in all of the other sources. The editor's redaction in the present edition (version "Dumitrescu") hence follows the German sources in this case and presents the Osanna sections in plainchant, continuing the alternatim structure present throughout the rest of the mass.⁴

Pre-existent material: Mass Ordinary plainchant for Eastertide, treated as cantus firmus in Discantus or Tenor, occasionally paraphrased or imitated in other voices

Graduale Pataviense (Vienna, 1511), ff. 184v-185v (Kyrie and Gloria), 194v (Sanctus and Agnus dei)

Editorially supplied material: Plainchant of alternate sections drawn from Graduale Pataviense (Vienna, 1511), ff. 184v-185v (Kyrie and Gloria), 194v (Sanctus and Agnus dei)

Notes
[1] Isaac, of course, did exceptionally produce a number of other six-voice alternatim masses: settings of the Missa de apostolis, Missa de beata virgine, and Missa solemnis.
[2] Even if the cantus firmus of Isaac's work is presented usually in the Discantus instead of the Tenor. In this sense the setting combines tenor-based structural techniques with the long-established tradition of placing Mass Ordinary paraphrases in the highest voice.
[3] Unmeasured final longa not included.
[4] The Agnus dei could be performed feasibly without plainchant, by performing its single section three times, the last time replacing the text "miserere nobis" with "dona nobis pacem." In the plainchant, all three sections have essentially the same music beginning at "qui tollis." The different openings of Agnus I and Agnus II/III (setting the words "Agnus dei") are both paraphrased simultaneously in the polyphony, Tenor and Bassus 2 performing Agnus I while Discantus 2 and Bassus 1 (Vagans) perform Agnus II/III. Isaac seems thus to have provided for multiple performance possibilities within this single section, in itself a minor combinatorial feat.