Medieval music was based on plainsong. This was a melody which sounded quite free in rhythm. Composers started adding a second part to the melody as an accompaniment. This was called organum. Sometimes it just simply followed the main tune a fourth or fifth below. This was called “parallel organum”. In the 12th and 13th centuries the original plainsong started to be put at the bottom. It became known as the cantus firmus (the “firm tune”). Sometimes antiphony was used. The simplest form of antiphony is when a leader sings something and a group (the choir) sing something back.