Gregorian chant is a type of sacred, liturgical music that was codified by Pope Gregory. This usually consists of a single line of music, sung by a group of monks, to accompany the liturgy. Gregorian chant remained the core of sacred music in the Western branch of the Christian Church for centuries, and has roots as far back as the fifth century A.D., although much of the music was not notated until much later, being passed through oral tradition (in music styled viva voce).
Gregorian chants are divided into eight modes, of which four are called “authentic” and the four others that are based on them are called “plagal.” Melodies whose final note is in the middle of the range of notes used, or which have only a limited range of notes, are categorized as plagal, and the others are called authentic. Each of these modes uses the hexachord system, and so modulation between the different modes is very easy.
Gregorian chants were notated with square notes called neumes, and written on to a staff of one or more lines (the modern five-line staff did not develop until the sixteenth century). Once the notation system had been developed, Gregorian chants quickly spread across Western Europe, displacing other forms such as Roman chant, Carolingian chant, Gallic chant, and Mozarabic chant. This spread shows how remarkably fast the dissemination of information could be in Medieval Western Europe, but fortunately the other forms of chant have survived in notation, although today their interpretation has not yet been decisively agreed-upon.