Organum is a plainchant-based musical style. At least one other voice sings along to enhance the harmony while one voice sings the primary chant melody. This style is significant to musicians, particularly music theorists, because it paved the way for true counterpoint to emerge.
Prior to 1000 A.D., there was an early organum. The Gregorian chants of the Catholic Church were the main source of inspiration for early works in this style. The vocal lines all moved in the same direction, so it was primarily parallel in structure. The non-chant voice, the vox organalis, was usually transposed into the chant line, the vox principalis, by a consonant interval.
Because consonant harmonies were required, most early organum used octaves, fourths, and fifths. Rather than being written down, the vox organalis lines were usually performed by trained singers who knew how to build simple harmonies “by ear.” As a result, the works were intended as reinforced single melodic concepts rather than true polyphony or multi-voiced music. However, because this reinforcement was seen as more glorious or complex than a single line, musicians frequently used organum to highlight special parts of the liturgy.
Composers began to push previously accepted musical boundaries during the Medieval period in order to create more complex “free” organum. Experimentation with oblique and contrary motion was a major development not long after the first millennium. The vox organalis moved away from the vox principalis line in an oblique motion. Both lines moved away from each other in opposite directions. With this advancement, true melodic independence in each musical line became possible, paving the way for more modern counterpoint.
The development of “florid” or “melismatic” organum peaked around the 12th century. The vox organalis can have up to six notes for each single note of the vox principalis in this style. The values of the notes in the chant melody were extended and became more like a drone as a result of this type of harmonization, with the elaborate singing in the vox principalis building up to the harmonic changes, while still moving naturally along. The note-against-note styles were dubbed discant, while the new style was dubbed “organum purum,” “organum duplum,” or simply “organum” to distinguish it from older styles.
During the florid period, there were two major schools of organum composition: the Saint-Martial of Limoges school and the Notre Dame of Paris school. Organum writing became increasingly refined and formalized as a result of these schools. Léonin, or Leoninus, and his successor, Pérotin, or Perotinus, were unquestionably the most important composers from these schools. It wasn’t uncommon for an organum to have at least three or four distinct parts by the time Pérotin was writing chant. Composers who came after had all the tools they needed to write contrapuntal vocal and other music, including the ability to use any type of musical motion, write down both melodic and harmonic concepts, and assign a line to each vocal type for increased range and complexity.