Known as Gregorian chant in its final form, plainsong is the traditional ritual melody of the Western Christian Church. Plainsong comprises a single unaccompanied line of vocal melody sung in free rhythm (not divided into bar-lengths). Plainsong also has its own system of notation employing a stave of four lines instead of five. Developed during the earliest centuries of Christianity (influenced by the Greek modal system, and possibly by the music of the Jewish synagogue), plainsong falls into two essentially distinct groups: the responsorial, which was developed from recitation of psalms around a dominant and antiphonal, developed as pure melody. A major reform of plainsong was initiated in the 6th century at the request (so it is said) of Pope Gregory. Hence the name Gregorian chant. In the music of the church and later in Western art music, plainsong formed the basis of cantus firmus.