we got both kinds of music, Organum AND Polyphony!

Organum (pronounced /ˈɔrɡənəm/, though the stress is sometimes placed on the second syllable, from Ancient Greek ὄργανον - organon "organ, instrument, tool" [1]) in general is 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 bourdon may be sung on the same text, or the melody is followed in parallel motion (parallel organum) or a combination thereof. 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 often the composition 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 proceed 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.