Gregorian chants are composed exclusively as vocal pieces; they are not played on any instruments. Melodies that include characteristics of a Gregorian chant and performed on a musical instrument or with vocal harmonies are not considered a Gregorian chant.
Gregorian chants are composed using modal scales. This is a distinct scale that is equivalent to musical compositions using only the white keys on a piano. The major and minor scales we are most familiar with did not exist when Gregorian chants first emerged.
Gregorian chant music is composed on staves containing only four lines instead of the conventional five-line staves on which other music is written. In addition, Gregorian chants do not use whole, half, quarter and eighth notes typically found in musical notation.
Gregorian chant melodies consist of a string of single notes sung with no harmonies. They are not accompanied by any additional voices or musical instruments.
Choirs singing Gregorian chants do not split into soprano, alto, tenor and bass voices. All voices sing the chant melodies in unison.