Sometimes inaccurately called Gregorian chant, but plain chant is a modification of the Gregorian, introduced among the Franks in the ninth century. It is unisonous and free-rhythmed and composed on a scale of four lines. The modes or scales of plain chant are limited to the natural intervals of the human voice and the melodies likewise confined to a person's natural range. Features of this chant are that the "leading note" is commonly avoided; also two choirs may be used while alternately singing the same melody. The beauty of plain chant derives from its pure melody, voice quality, and subtle rhythm.