Organum is a musical style based on plainchant. While one voice sings the primary chant melody, at least one other voice sings along to enhance the harmony. This style is important to musicians, particularly music theorists, because it served as the basis for the development of true counterpoint.
Early organum was present prior to 1000 A.D. Works in this early style developed mainly from the Gregorian chants of the Catholic Church. It was primarily parallel in structure, meaning that the vocal lines moved in the same direction. The added non-chant voice, the vox organalis, usually was transposed by a consonant interval with the chant line, the vox principalis.
Most early organum used octaves, fourths and fifths as a result of the requirement for consonant harmonies. The vox organalis lines typically were not written down and instead were performed by trained singers who understood how to construct the simple harmonies "by ear." Thus, the works were intended not as true polyphony, or multi-voiced music, but merely as reinforced single melodic concepts. This reinforcement was seen as more glorious or complex than one line alone, however, so musicians often used organum to highlight exceptional portions of the liturgy.
Through the Medieval period, composers began to push previously-accepted musical boundaries to develop more complex "free" organum. A major development that happened not long after the first millennium was experimentation with oblique and contrary motion. In oblique motion, the vox organalis moved away from the vox principalis line. In contrary motion, both lines moved apart from each other. With this development came the possibility of true melodic independence in each musical line, which set the stage for more modern counterpoint.
Organum peaked around the 12th century with the development of "florid" or "melismatic" organum. In this style, the vox organalis to have up to six notes for each single note of the vox principalis. The result of this type of harmonization of the chant was that the values of the notes in the chant melody, although still moving naturally along, were extended and became more like a drone, with the elaborate singing in the vox principalis building up to the harmonic changes. To distinguish between this newer method and older styles, the note-against-note styles were called discant, while the new style was called "organum purum," "organum duplum" or simply "organum."
Two major schools of organum composition during the florid period were the Saint-Martial of Limoges school and the Notre Dame of Paris school. It was through these schools that organum writing became increasingly refined and formalized. In terms of composers from these schools, probably the most significant individuals were Léonin, or Leoninus, and his successor, Pérotin, or Perotinus. By the time Pérotin was writing chant, it wasn't unusual for an organum to include at least three or four distinct parts. With the ability to use any type of musical motion, write down both melodic and harmonic concepts and have a line assigned to each vocal type for increased range and complexity, composers who followed had all the tools they needed to write contrapunctal vocal and other music.