The official monophonic unison chant, originally unaccompanied, of the Christian liturgies. The term refers particularly to the chant repertoires with Latin texts, i.e. those of the major Western Christian liturgies (Ambrosian, Gallican, Mozarabic and Gregorian and Old Roman); and in a more restricted sense to the repertory of Gregorian chant, the official chant of the Roman Catholic Church.

The origins of Christian liturgical chant lie in Jewish synagogue practice and in pagan music at early church centres (Jerusalem, Antioch, Rome and Constantinople). By the end of the 4th Century there were distinct families of Eastern and Western (Latin) rites, each with its own liturgy and music. As political and liturgical unification began under Carolingian rile in the mid-8th century, all the local Latin musical rites, except the Ambrosian were supressed in favour of the Gregorian. Notation appears nowhere before the 9th century, precise pitch representation being found only a century or two later. Of the Latin rites, only the Gregorian, Old Roman and Ambrosian survive completely.

Each plainchant family has its distinctive modal idioms; in some repertoires (Gregorian-Old Roman, Byzantine, Slavonic, Coptic) the modes are assigned numbers or names. The Byzantine modal theory Oktoechos developed with a symmetrical arrangement of eight modes and was adopted by the Gregorian repertoire of the 8th century. These use four final pitches (D, E, F and G), with sub-forms in a higher range (authentic) and lower range (plagal) for each final. Certain modes are preferred for certain liturgical seasons or particular feasts. In the Gregorian tradition tonaries from the 9th century onwards listed melodies by mode, imposing the modal system only after the repertory had been fixed.

The forms or the chant repertory can be divided into psalmodic and non-psalmodic. These are three main forms of psalmody: antiphonal, in which two halves of a choir sing psalm verses in alternation with a refrain (antiphon); responsorial, in which one or more soloists alternate with the choir in singing psalm verses and a refrain (respond); and a direct, in which the cantors sing verses without a refrain.

Non-psalmodic forms include the strophic form of the hymn, in which a single melody is repeated for all strophes; the sequence, in which there is repetition within each couplet; the repititive forms of Kyrie and Agnus Dei; and the non-repetitive forms of the Sanctus, Gloria and Credo. In the Mass, the chants of the Ordinary are all non-psalmodic and those of the Proper are psalmodic. Recitation formulae are used for both psalmodic and non-psalmodic texts. The syllabic psalm tones are the musical patterns based on mode that accommodate the recitation of psalm verses. The beginning, middle and end of each verse are punctuated with small intonation, flex, mediant and cadential formulae.

There are three melodic styles of chant: syllabic, in which each syllable of text is set to a single note; neumatic, in which two to a dozen notes accompany a syllable; and melismatic, in which single syllables may be sung to dozens of notes. The Christian liturgies are divided into the Eucharistic Mass and the Divine Office, and it is the liturgy that determines the musical style of plainchant. In general, the more solemn the occasion, the more florid the music, although the most solemn chants are intoned by the celebrant. Each family of chant is characterized by a specific melodic type: antiphons and psalms are normally set syllabically, introits, Sanctus and Agnus Dei melodies are neumatic, and graduals, alleluias and offertories contain extensive melismas.

Chant composition involves the contrived selection of traditional modal materials, which may be divided into cells, formulae and patterns. Cells are miniature melodic gestures, which either stand alone or contribute to the larger stylized formulae; formulae are longer, more individual melismatic elements; and patterns are flexible frameworks or pitches that accomodate whole phrases of text. These melodic idioms are chosen and ordered according to established modal procedures.

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