Organum is a type of music that begins with a plainchant melody and adds at least one more voice to create harmony. It was developed during the Middle Ages. Depending on the style of the chant, a supporting bass line (called a bourdon) may be sung using the same text, the melody may be repeated in parallel motion (called parallel organum), or both techniques may be used together. Since the added voice does not fully separate from the original melody, this is a type of heterophony. In its earliest form, organum used two voices: one was a Gregorian chant melody, and the other was the same melody shifted up by a consonant interval, such as a perfect fifth or fourth. These compositions often started and ended with both voices singing the same note. The added voice stayed on the starting note until the first voice reached the fifth or fourth, after which both voices moved together in harmony, repeating the process in reverse at the end. Organum was originally created by improvising. One singer followed a written melody (called the vox principalis), while another singer (called the vox organalis) added a second melody without written notes. Over time, composers began writing more complex added parts, leading to true polyphony.

History

The first document that specifically described organum and provided rules for how it should be performed was the Musica enchiriadis (around 895). The oldest methods of teaching organum can be found in the Scolica and the Bamberg Dialogues, along with the Musica enchiriadis. Societies that developed polyphony usually had several types of it in their culture. Originally, organum was not intended as polyphony in the modern sense; the added voice was meant to strengthen or enhance the plainchant during important religious celebrations to make the liturgy more grand. The development of sacred architecture and music shows a similar pattern: in earlier times, monophonic Masses were held in abbatial churches; by the 12th and 13th centuries, newly built cathedrals echoed with increasingly complex forms of polyphony. While the exact timeline and locations of these developments are sometimes unclear, some key points remain visible in written treatises. It is difficult to determine whether these treatises describe actual practices or deviations from them. A key idea behind the creative growth in the 11th and 12th centuries was the addition of harmonic layers to music, as the rich harmony of organum increased the grandeur and solemnity of religious celebrations.

The earliest European sources about organum describe it as a common practice. Organum was performed in various religious traditions, but most information about its history comes from Gregorian chant. Since trained singers had learned an oral tradition that was hundreds of years old, singing parts of the chant repertoire in simple parallel harmony or other ways of "singing by ear" was natural. The Musica enchiriadis explains that doubling a melody by an octave (called magadization) was acceptable, as this often happened when men and boys sang together. The 9th-century treatise Scolica enchiriadis discusses this topic in more detail. In parallel singing, the original chant was the upper voice (vox principalis), and the added voice (vox organalis) was sung at a perfect interval below, usually a fourth. This made the original melody the main voice, while the added voice acted as an accompaniment or harmonic support. This type of organum is sometimes called parallel organum, though early writings used terms like sinfonia or diaphonia.

The history of organum would not be complete without two important innovators: Léonin and Pérotin. These two men were the first international composers of polyphonic music. Their work marked the development of rhythmic modes. Their innovations were based on Gregorian chant and followed the rhythmic systems described by St. Augustine. Their love for the cantus firmus (a fixed melody) kept the tenor line (the main melody) unchanged, even as musical notation evolved. However, their use of modal rhythm—a pattern of notes with unequal lengths—was key to their success. The development of rhythmic modes by the Notre-Dame composers allowed music to become independent from the text. While Léonin composed many pieces of organum, it was Pérotin’s revisions of Léonin’s organum purum that influenced future composers of organum and motets to use rhythmic modes.

Cultural and intellectual life in Paris flourished in the 12th century, especially after the University of the Sorbonne became a respected institution that attracted students from many countries. The construction of Notre-Dame Cathedral on the Île de la Cité (1163–1238) coincided with the development of the Paris style of organum. The cathedral and the University of Paris became centers for musical composition and the teaching of musical theory in the 12th and 13th centuries. The presence of Léonin and Pérotin at the Notre-Dame School made Paris the heart of musical innovation in the 12th century. Léonin, the magister cantus (master of music) at Notre-Dame, compiled the Magnus Liber Organi de Gradali et Antiphonario (The Great Book of Organum for the Gradual and Antiphonary). He wrote organa dupla (two-part organum) based on existing chants like the Alleluia and Gradual from the Mass and Benedicamus Domino from Vespers for major religious ceremonies. In hindsight, this was a major event, as it was the first large-scale project attributed to a single composer. It was not only a practical guide for use during Mass and other religious services but also introduced the use of rhythmic modes as a creative principle. When discussing organum from the Paris School, the word "modal" or "mode" refers to rhythmic modes, not the musical modes that shape melodies.

In Léonin’s Organa de Gradali et Antiphonario, two forms of organum are clearly shown: organum purum and discantus. Benedicamus Domino is a good example of these techniques. The word Benedicamus is usually syllabic (one note per syllable) and is set in florid organum (a flowing, melodic line) over a sustained tenor. The word Domino is set in a melismatic style (multiple notes per syllable) and uses discantus, where both the tenor and the added voice move in rhythm based on the six rhythmic modes, ending with a florid cadence over a sustained tenor. In larger texts, syllabic sections (without ligatures and therefore non-modal) become organum purum, where the tenor holds each note of the chant while the added voice creates a new, flowing line written mostly in ligatures and compound neumes. Starting from a consonant (often an octave), the added voice explores harmonic interplay with the tenor, building up to a change in harmony at the end of a melisma (a group of notes on a single syllable) where another syllable is sung at a different pitch. When the chant uses ligatures or melismas, both voices move in a rhythmic mode. This section of discantus ends, on the last syllable of a word or phrase, with a copula, where the tenor holds either the penultimate or final note, and the added voice returns to