Organum

Organum (/ˈɔːrɡənəm/) is, in general, a plainchant melody with at least one added voice to enhance the harmony, developed in the Middle Ages. Depending on the mode and form of the chant, a supporting bass line (or bourdon) may be sung on the same text, the melody may be followed in parallel motion (parallel organum), or a combination of both of these techniques may be employed. As no real independent second voice exists, this is a form of heterophony. In its earliest stages, organum involved two musical voices: a Gregorian chant melody, and the same melody transposed by a consonant interval, usually a perfect fifth or fourth. In these cases the composition often began and ended on a unison, the added voice keeping to the initial tone until the first part has reached a fifth or fourth, from where both voices proceeded in parallel harmony, with the reverse process at the end. Organum was originally improvised; while one singer performed a notated melody (the vox principalis), another singer—singing "by ear"—provided the unnotated second melody (the vox organalis). Over time, composers began to write added parts that were not just simple transpositions, thus creating true polyphony.

The first document to describe organum specifically, and give rules for its performance, was the Musica enchiriadis (c. 895), a treatise traditionally (and probably incorrectly) attributed to Hucbald of St. Amand. The oldest methods of teaching organum can be found in the Scolica and the Bamberg Dialogues, along with the Musica enchiriadis. The societies that have developed polyphony usually have several types of it found in their culture. In its original conception, organum was never intended as polyphony in the modern sense; the added voice was intended as a reinforcement or harmonic enhancement of the plainchant at occasions of High Feasts of importance to further the splendour of the liturgy. The analogue evolution of sacred architecture and music is evident: during previous centuries monophonic Mass was celebrated in Abbatial churches, in the course of the 12th and 13th centuries the newly consecrated cathedrals resounded with ever more complex forms of polyphony. Exactly what developments took place where and when in the evolution of polyphony is not always clear, though some landmarks remain visible in the treatises. As in these instances, it is hard to evaluate the relative importance of treatises, whether they describe the 'actual' practice or a deviation of it. As key-concept behind the creative outburst that manifested in the 11th and 12th centuries is the vertical and harmonic expansion of dimension, as the strongly resonant harmony of organum magnified the splendour of the celebration and heightened its solemnity.

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