Plain Chant
Plain song. Cantus planus, or Cantus choralis (Lat.), the early music of the church, written in the ecclesiastical modes (also called Ambrosian) and Gregorian scales. In the 12th century the unrhythmic melodies of the early forms of plain song were largely superseded by the rhythmic cantus mensurabilis, or measured song, which came into existence upon the invention of notes by Franco of Cologne.
Before this invention the musical rhythm depended entirely on the rhythm of the words to which it was sung.