In origin, it is a style of organum that either includes a plainchant tenor part (usually on a melisma in the chant) or is used without a plainchant basis in conductus, in either case with a "note against note" upper voice, moving in contrary motion. It is not a musical form, but rather a technique. The term continued to be used down to modern times with changing senses, at first for polyphony in general, then to differentiate a subcategory of polyphony (either in contrast to organum, or for improvised as distinct from written polyphony). By extension it became the name of a part that is added above the tenor, and later as the name of the highest part in a polyphonic setting (the equivalent of "cantus", "superius", and "soprano"). Finally, it was adopted as the name of the highest register of instruments such as recorders, cornets, viols, and organ stops.