vox principalis

…an existing plainchant melody (the vox principalis, or principal voice), which by the end of the 12th century was stretched so as to accommodate a melody. The 13th-century polyphonic motet, for its part, featured the plainchant cantus firmus in the tenor. (“Tenor” derives from Latin tenere, “to hold”—i.e., the voice…

… melody, or “principal voice” (vox principalis), is combined with another part, “organal voice” (vox organalis), singing the same melody in parallel motion a perfect fourth or fifth below (e.g., G or F below C).

To a given plainsong (or vox principalis), a second voice (vox organalis) could be added at the interval (distance between notes) of a fourth or fifth (four or five steps) below. Music so performed was known as organum. While it may be assumed that the first attempts at polyphony involved…