Organum is an early Medieval form of plainsong, or plainchant (e.g. Gregorian chanting) that has it's origins from 9th Century France. In it's beginnings, organum was sung with at least one voice added to create harmony, usually a perfect fifth or fourth. Organum was originally improvised with one singer performing a written melody (the vox principalis), and a second singer improvising the harmony (the vox organalis) in fifth and fourth intervals. Using this method, this form was considered heterophonic, meaning only one voice or melody was carried by the song. True polyphony was reached later through the development of parallel oganum and then free organum which utilizes parallel motion and oblique motion (upper voice moving while the tenor holds one note). Organum can be said to have graduated into florid organum which uses anywhere from two to six notes in the organal voice sung over a single sustained note by the tenor voice. <br>