The title "ad organum" is generally applied to this mass on the evidence of its one complete source, VienNB Mus. 18745, and in this capacity serves handily to distinguish it from Isaac's other Paschal masses (including one other four-voice setting). The distinction, however, is very likely a modern one, reflecting the idiosyncracies of the source situation rather than a difference of compositional or performance style, as alternation between organ versets and composed vocal polyphony seems to have been the general practice for Isaac's alternatim masses in Maximilian I's chapel where he served.¹

The Kyrie of this mass was transmitted separately outside German territories in one surviving manuscript, the Occo Codex from the Habsburg-Burgundian court scriptorium, pre-dating the other sources by a considerable margin. There it occupies an interior position within an extended version of Isaac's six-voice Paschal mass, serving along with Vourda's Kyrie Paschale as one of three alternative Kyries in a unit which the scribe titled "Missa paschale sex vocum. heinricus ysaac. &c." (see the Introduction to the Occo Codex edition). As in many settings of items from the Easter mass, plainchant is foregrounded through quotation often in the highest voice rather than the Tenor: only at the close of the Criste does the Tenor begin to assume a cantus firmus role, and even this voice's complete presentation of the melody of the final Kyrie serves as a secondary canonic follower to the Discantus where the plainchant is notated in breves and longae to be sung at double-speed (mm. 49-61). The primacy of the top voice in presenting chant materials connects this (and other Paschal settings) more directly to a long tradition of "functional" polyphony than to the artful Tenor-mass, even if Isaac shows no hesitation in importing a sophisticated modern contrapuntal style into this context. Particularly notable in the present setting is the employment of brief characteristic motives as unifying elements throughout a section and not merely as imitative units (e.g., the opening motive announced in the Contratenor in m. 2 which begins to saturate the texture only in m. 8).

Pre-existent material: Plainchant Kyrie Lux et origo, treated as cantus firmus in Discantus or Tenor, occasionally paraphrased or imitated in other voices

Graduale Pataviense (Vienna, 1511), ff. 184v-185v

Editorially supplied material: Plainchant of alternate sections drawn from Graduale Pataviense (Vienna, 1511), ff. 184v-185v

Notes
[1] See William P. Mahrt, The 'Missae ad Organum' of Heinrich Isaac (Ph.D. dissertation, Stanford University, 1969).