Cantus firmus treatment

Voices

Although it is usually possible to identify the principal cantus-firmus voice or voices at any given time, the cantus firmus migrates frequently among voices. It is usually imitated in at least one other voice, and it is often in canon in two or more voices.

Rhythm and ornamentation

In the sequences, which are based on syllabic chants, the cantus firmus is most often in consistent rhythmic values. Ornamentation is modest, and ornamental notes are concentrated at ends of phrases and in the final phrases of verses. Verses in duo texture are an exception: they are often highly ornamented and imitative to the extent that it is not possible to identify either voice as the principal bearer of the cantus firmus.

The most common basic rhythmic value for the cantus firmus in the sequences is the semibreve, sometimes preceded by breves for the first few notes. Other values may be interspersed within this basic pattern. In perfect tempus and sesquialtera, the most common cantus-firmus rhythm is alternating breves and semibreves.

The following verses feature unusual rhythmic values for the cantus firmus:

• Longs preceded by a maxima: Sequence 11, v. 12
• Breves preceded by a long in each phrase: Sequence 10, v. 14
• Breves: Sequence 3, v. 4; Sequence 8, v. 24; Sequence 12, v. 10; Sequence 12, v. 16; Sequence 12, v. 18; Sequence 14, v. 14; Sequence 17, v. 8
• Perfect breves: Sequence 21, v. 14; Sequence 3, v. 14 (notated as semibreves in augmentation)
• Dotted semibreves in perfect tempus: Sequence 8, v. 14
• Dotted semibreves in imperfect tempus: Sequence 16, v. 18
• Syncopated semibreves: Sequence 15, v. 6
• Semibreve-minim pairs imperfect tempus: Sequence 20, v. 2
• Perfect breve-imperfect breve-semibreve groups in perfect tempus: Sequence 16, v. 16
• Perfect breve-imperfect breve pairs in perfect tempus or sesquialtera: Sequence 8, v. 18
• Seven minims (notated as dotted breve + minim pairs, since a 7-minim value cannot be notated in imperfect tempus): Sequence 16, v. 4

Multiple cantus firmi

Isaac's techniques for combining cantus firmi are impressively clever and varied:

• Introit 6 (antiphon section) combines the popular song Christus surrexit with the Easter introit Resurrexi. The song enters before the chant paraphrase, no doubt startling the listeners who were expecting a setting of the traditional chant. It migrates through all the voices and reaches a climactic point when it is taken up by the discant in a register that is distinctly higher than anything heard previously. The chant is in mode 4, transposed to A in Isaac's setting. The final cadence has a supplementum bringing the bass and altus to D. The song is in mode 1 on G, which is compatible with the bulk of the chant, in which the pitches G, B-flat, and D predominate. At the end, the song is transposed up a fifth to end on D, matching the final sonority of the chant voices.
• Introit 13 (antiphon section) uses the opening word of the introit Gaudeamus omnes as an ostinato that migrates among all voices throughout the piece. The word is associated with a striking musical motive that rises an octave, from the subfinal to the seventh degree, then cadences on the fifth degree of the mode. It allows the exuberance of that motive (a standard opening figure in first-mode chants, used here with evident expressive effect) to permeate the entire piece.
• Introit 14 (antiphon section) combines a mostly monotone litany addressed to Saint Mary Magdalen with the introit Gaudeamus omnes. The first statement of the litany employs speech-like rhythms in semibreves and minims. Later statements feature unusual declamation with consecutive syllables on semiminims, highlighting the intensity of the prayer. The litany migrates among the lower three voices, leaving the discant free to present the principal cantus firmus as a contrasting element.
• Introit 15 combines the antiphon Virgo prudentissima with the introit Gaudeamus omnes. In the antiphon section of the introit, the Gaudeamus omnes chant remains in the discant with Virgo prudentissima in the tenor. In the verse, the voices exchange roles, metaphorically exalting the Virgo prudentissima to the top of the texture in response to the text: "Exaltata es, sancta Dei genitrix, super choros Angelorum ad coelestia regna."
• Introit 18 (antiphon section) combines the antiphon Cum iocunditate with the Introit Gaudeamus omnes. Cum iocunditate is stated twice: first in the altus, then in the bass. It ends with the words "ad Dominum Iesum Christum." The bass statement ends immediately before the final words of Gaudeamus omnes, "filium Dei," and dovetails perfectly with that text. Cum iocunditate is in mode 7 transposed to C. With an opening pitch G and cadences on G and C, it combines comfortably with the introit in mode 1 on G.
• Alleluia 13 (verse), which has the same words as the first verse of the Magnificat canticle, combines an ostinato figure from the recitation formula for the canticle with the Alleluia chant. The Alleluia is in mode 5 on F. Except in the first statement, the canticle formula that matches it contrapuntally is not the one in mode 5, but the one in mode 8, which is transposed to C with the reciting tone on F in most statements. In two statements, including the last one, it is transposed to F with the reciting tone on B-flat. This makes its last two pitches G-F, the required steps for the tenor in the final cadence of the piece on F.
• Alleluia 15 combines the antiphon Descendi in hortum nucum with the principal chant in both sections, with the first phrase in the Alleluia section and the remainder in the verse. The first phrase of the antiphon melody is nearly identical to the Alleluia melody up to the jubilus; the entry of the antiphon therefore sounds like a free imitation of the cantus firmus, even though it is a different piece. The second phrase of the antiphon melody is likewise similar to the beginning of the Alleluia verse and creates a similar effect of free imitation between the two independent melodies.
• Alleluia 24 (verse) combines the antiphon Ecce sacerdos magnus with the Alleluia chant. The alleluia has the same words as the antiphon with the words "pius Conradus" added at the end. Since we have not located the Alleluia chant, we cannot tell how the two melodies interact.
• Sequence 6, the Easter sequence Laudes salvatori, features the most elaborate combination of multiple cantus firmi in Choralis II. The Marian antiphon Regina coeli, which is sung seasonally beginning at Easter, is combined with the principal chant in verses 2-8; another Easter sequence, Victimae paschali, is combined with it in verses 12-22. Verse 10 is a duo without a second cantus firmus. Isaac makes these contrapuntal combinations work seamlessly, even though Laudes salvatori is in mode 7 on C, Regina coeli is in mode 5 on F, and Victimae paschali is in mode 1 on G. He coordinates the words and musical motives between the contrasting chants in subtle ways. Striking examples include:
• In verse 16, both chants end with the word "vivus," and thus appear to merge into one at that point.
• In the final verse (verse 22), the Victimae paschali chant is mostly in the discant, but the first two words are placed in the tenor. This makes it possible to reserve the high g" for the climactic words "Scimus Christum surrexisse a mortuis vere," which are set in even semibreves over fast runs on the word "resurgens" in the lower voices.