Which type of musical composition has two or more parts, one of which is an existing monophonic melody

Organum is musical composition consisted of two melodic lines moving simultaneously note against note , one of which is an existing monophonic melody (melody or “tune”, typically sung by single singer or played by single instrumental player). Organum earliest definition and teachings are found in “Musica enchiriadis”, “Scolica” and the “Bamberg Dialogues”. The best known composers of organum are Léonin and Pérotin.

the string with the finger instead of using the bow