Techniques for spicing up plainchant eventually led to polyphony: music with two or more independent lines. Without compromising the integrity and primacy of the chant line, medieval sacred music explored the following:
Responsorial (alternate soloist and group)Polyphony emerged in the 8th or 9th century and appears in an early elementary form in "parallel organum": chant with almost strictly parallel progression. Doubling the chant line in another octave does it no disservice, so why not at the fifth or fourth? It gives the music a fascinating resonance. Musica enchiriadis ("Musical Handbook") from the latter half of the 9th century provides our first examples of parallel organum. In some instance, the two voices start in unison and the vox organalis climbs to its interval, returning at the ends of phrases to the unison. Organum appeared in contrary motion too, a mirroring of lines leading to issues of counterpoint. "Oblique organum" refers to the practice of staying on a note to avoid the tritone. In "free organum" the organal voice appears above the tenor but may cross or mirror it.
Antiphonal (alternate between equal groups, monasteries were usually set up like this anyway)
Processional (movement so sound emanates from various places)
Octaves (boys in monasteries with higher voices)
Organum (natural divisions of octaves occur at fourths and fifths; to the "vox principalis" was added a line moving at a constant interval and called the "vox organalis").
Ad organum faciendum ("On the Making of Organum"), a treatise from circa 1100, shows that the "vox organalis" had become more ornate over the century and had taken a higher register than the original chant line. The chant was considered more or less sacrosanct, not to be altered, but the vox organalis was freer. When the lower chant notes were sustained longer and the upper voice became more elaborate, we have "melismatic organum." The chant phrase is called the "tenor" (from the Latin tenere, meaning "to hold"), later the "cantus firmus," and the vox organalis becames the "duplum" (second part).
Florid organum = melismatic organum = organum duplum = organum purum. Unmeasured melismatic duplum over long tenor notes appears in the 11th and 12th centuries. "Aquitanian organum" is associated, of course, with SW France. "Discant organum" refers to the two voices falling into a rhythmic mode -- a 6/8 or 9/8 feel -- singing more or less at the same rate for a passage. In the 11th and 12th centuries, octaves, fourths, and fifths were considered consonant; but not thirds yet. The 1-5-8 structure was the standard closing sonority since it is composed of two perfect intervals.
Paris was the cultural center in the late 12th and 13th centuries. Composers working at Notre Dame in Paris in the 12th century were responsible for the Magnus liber organi ("The Big Book of Organum"), and an English student at the University of Paris known later only as Anonymous IV reports that the great innovator behind the project was Léonin, surpassed only by Perotin later who revised the book. Léonin's contributions appear as "free organum" (or "unmeasured organum") or as "discant organum" ("measured organum"). In the latter, the two voices move at about the same pace. Perotin is credited with having added a third voice called the "triplum" and sometimes even a fourth, the "quadruplum." He also used more measured rhythms in the upper voice against sustained tenor notes. These polyphonists enjoyed a somewhat codified rhythmic system: six patterns roughly aligned with the poetic patterns known as trochaic, iambic, dactylic, anapestic, spondaic, and tribrachic. "Ligatures" -- symbolic groupings of a few notes -- identified these rhythms. Triple divisions were considered "perfect" due to the pervasive notion of the Holy Trinity, while binaries were considered imperfect.
Franco of Cologne, a German theorist and composer, was responsible for Ars Cantus Mensurabilis (c.1280), setting choirbook format to save paper where motetus on the left of the page and triplum on the right appeared above the tenor, running along the bottom with its typically long notes. Franconian notation involved long notes (like flags, signifying a dotted half note or half note, depending on the context), breves (blocks equal to a quarter note), and semibreves (diamonds equal to eighth notes).
Petrus de Cruce (Pierre de la Croix) was a French composer (active 1270-1300) who provided for faster note values: the minim (a diamond with a stem) and semiminim (a tail on the stem). Towards increasingly useful notation was use of a "punctus divisionis," a dot to show how rhythmic groupings will go.
Philip de Vitry (1291-1361) would coin in his treatise the term "Ars Nova," meaning this 13th-century polyphony was "Ars Antiqua."