or plainchant, the unharmonized chant of the medieval Christian liturgies in Europe and the Middle East; usually synonymous with Gregorian chant, the liturgical music of the Roman Catholic Church.

In the Western church four main dialects of plainsong developed—Ambrosian, Roman, Mozarabic and Gallican—that seem to have been derived from similar sources. Gregorian chant is named for Pope Gregory I, whose credited role in compiling liturgical books during his papacy (590–604) is now considered questionable.

The origins of the chant go back to early Christian times, and it seems to have derived from musical practice in the Jewish synagogue and Greek musical theory. During the late Middle Ages and the Renaissance, and also in later times, the chant melodies were used as the basis for polyphonic composition. In the 19th cent. the Benedictine monks of Solesmes sought to restore the Gregorian chant to its original form and their published editions from 1889 onward became the official music of the Catholic Church. The texts of plainsong are the words of the Mass, the Psalms, canticles, and certain verse hymns.

The tonality of Gregorian chant is based on the system of eight modes (see mode). The notation of the chant evolved into systems of neumes (see musical notation) that were still used in the 20th cent. in preference to modern mensural notation for plainsong. Little is known of the rhythm with which the chants were performed in the Middle Ages. The chants were contained in two principal books: those for the Mass in the "Gradual," those for the Office in the "Antiphoner." The modern Liber usualis is a compilation of most frequently used chants from the two.

See W. Apel, Gregorian Chant (1958); J. R. Bryden and D. G. Hughes, ed., An Index of Gregorian Chant (2 vol., 1969).

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