Plainsong (also plainchant; Latin: cantus planus) is a body of chants used in the liturgies of the Western Church. Though the Catholic Church (both its Eastern and Western halves) and the Eastern Orthodox churches did not split until long after the origin of plainsong, Byzantine chants are generally not classified as plainsong.
Plainsong is monophonic, consisting of a single, unaccompanied melodic line. Its rhythm is generally freer than the metered rhythm of later Western music.
Plainsong developed during the earliest centuries of Christianity, influenced possibly by the music of the Jewish synagogue and certainly by the Greek modal system. It has its own system of notation, employing a staff of four lines instead of five.
Two methods of singing psalms or other chants are responsorial and antiphonal. In responsorial singing, the soloist (or choir) sings a series of verses, each one followed by a response from the choir (or congregation). In antiphonal singing, the verses are sung alternately by soloist and choir, or by choir and congregation. It is probable that even in the early period the two methods caused that differentiation in the style of musical composition which is observed throughout the later history of plain chant, the choral compositions being of a simple kind, the solo compositions more elaborate, using a more extended compass of melodies and longer groups of notes on single syllables. A marked feature in plain chant is the use of the same melody for various texts. This is quite typical for the ordinary psalmody in which the same formula, the "psalm tone", is used for all the verses of a psalm, just as in a hymn or a folk song the same melody is used for the various stanzas.
Gregorian chant is a variety of plainsong named after Pope Gregory I (6th century A.D.), although Gregory himself did not invent the chant. The tradition linking Gregory I to the development of the chant seems to rest on a possibly mistaken identification of a certain "Gregorius", probably Pope Gregory II, with his more famous predecessor.
For several centuries, different plainchant styles existed concurrently. Standardization on Gregorian chant was not completed, even in Italy, until the 12th century. Plainchant represents the first revival of musical notation after knowledge of the ancient Greek system was lost. Plainsong notation differs from the modern system in having only four lines to the staff and a system of note shapes called neumes.
There was a significant plainsong revival in the 19th century, when much work was done to restore the correct notation and performance-style of the old plainsong collections, notably by the monks of Solesmes Abbey, in northern France. After the Second Vatican Council and the introduction of the vernacular Mass, use of plainsong in the Catholic Church declined and was mostly confined to the monastic orders and to ecclesiastical societies celebrating the traditional Latin Mass (also called Tridentine Mass). But, since Pope Benedict XVI's motu proprio, Summorum Pontificum, use of the Tridentine rite has increased; this, along with other papal comments on the use of appropriate liturgical music, is promoting a new plainsong revival.
The Plainsong and Medieval Music Society was founded in 1888 to promote the performance and study of liturgical chant and medieval polyphony.
Interest in plainsong picked up in 1950s Britain, particularly in the left-wing religious and musical groups associated with Gustav Holst and the writer George B. Chambers. In the late 1980s, plainchant achieved a certain vogue as music for relaxation, and several recordings of plainchant became "classical-chart hits".