A cantus firmus is also referred to as a "fixed song." It is a melody that comes from a chant and forms the basis of a polyphonic composition and is performed in long notes. The chant was most popular in the 14th and 15th centuries. There are over 38 settings that are known, including some by Josquin des Prez, and some by an anonymous composer or composers in Naples.
Around 900 AD the polyphonic compositions contained the chant in a high voice with a new composed part underneath. During the 1100 the composition changed and the cantus firmus appeared in a low sounding voice. The cantus firmus later changed to the tenor voice which allowed notes for holding longer.
Cantus firmus was used as a teaching tool by many composers. It was the basis for the Gradus ad Parnassum by Johann Joseph Fux. The method was first published and produced by Girolamo Diruta in 1610. The counterpoint of the music is still taught and based on the cantus firmus.
Cantus firmus continued through to the 13th century. Nortre Dame and St. Martial schools use the cantus firmus music. The motets were written in many languages and the cantus firmus is in the lowest voice. Around the 15th century the cantus firmus technique became the common organizing principle. It was restricted at first to just the tenor, but later many composers experimented with other ways to use it.
By the 16th century the cantus firmus technique was not use as much. It was replaced with the parody, which was known as the imitation technique. Multiple voices of sources that already existed were for composition. Composers in Germany, Portugal and Spain continued to use the cantus firmus. Well-known composers in the Baroque period such as Bach use chorale melodies as cantus firmus.