PLAIN SONG (Plain Chant, Cantus planus, Gregorian Chant), the most ancient and sim ple form of church music, consisting of easy progressions in one of the church modes, suit able for use by priests or a congregation. When counterpoint was introduced, it was cus tomary to compose parts above or below a por tion of ancient plain song; hence the term plain song is often synonymous with canto fermo, or the fixed melody to which counterpoint is added. It differs from modern music, (1) in its recitativo-like character, as opposed to barred music; it has no marked and regular rhythm; the rhythm of a piece of modern music is indicated by the signature as being either in common or in triple time or some vari ety of these, and the stave is divided into bars or passages equivalent in length; but in plain song there is no such division, and the rhythm of the music is derived from the metrical rhythm of the psalm or hymn rather than vice versa. (2) The modes or scales in which plain song is written are more numerous and varied than the modern major and minor. (3) The notation of plain song is by means of a stave of four lines, on any one of which either of the two received clefs (C and F) may be set and so determine the sound of all other notes, above and below. Originally the notes in plain song were all of one length; they are now of three kinds, longs, breves and semibreves, the long being equal to the breve and semibreve. The modes of plain song are named after the an cient Greek modes — Dorian, Phrygian, Lydian, etc., but beyond the name there is no relation between the two systems. The principle of the plain song modes is that each of the seven nat ural sounds of the diatonic scale forms the key note or "final" of a mode, which comprises that note and the seven above it. (The melodies rarely exceeded an octave, and no flats or sharps are used except an occasional B flat). This gives seven modes; but to each of these is attached another, in which the melody, while having the same "final" or key-note, instead of rising to the octave above, ranges from the fourth below it to the fifth above; these are the authentic and the plagal modes respectively. The difference between an authentic and a pla gal melody may be illustrated from the two psalm tunes Newton or New London and the Old Hundredth; in the first the melody lies be tween the key-note and its octave, but in the other between the fourth below and the fifth above the key-note. Of these 14 modes two
are universally rejected in practice as defective — the llth and 12th. In the following table the notes of the scale are indicated by the letters of the alphabet according to modern notation; the "final" or key-note in each mode is distin guished by full-face type.
The Ionian is the modern major mode. In one or other of the 12 approved modes every plain song melody is composed. The psalm tunes are written in the eight modes one, three, five and seven (authentic), and two, four, six and eight (plagal); the 114th psalm, In Esau Israel, has from ancient times been sung to a peculiarly beautiful melody in mode nine, the Tonus peregrinus: ancient tradition traces this tune back to the time of the Maccabees and the purification of the Temple after the first victo ries of Judas Maccabmus (165 ac) on which occasion were sung the psalms of the Hallel (Ps. cxiii-cxviii); and it is highly probable that all the psalm tones used by the church had their origin in the Jewish Temple-service. From the beginning vocal music was employed in the church service and formed an important part of it. At first the tones and melodies were handed down orally, afterward some crude sys tem of notation was devised, but it was defect ive, and was reformed by Saint Ambrose, bishop of Milan (4th century), who brought to the West the mode of chanting which he had learned during his residence at Antioch: the (Ambrosian chant' was founded on the first four authentic modes, and was sung antipho nally. Till the pontificate of Gregory I it was widely in use in the Western churches; but after Gregory's reform the Ambrosian chant was disused, and is now practised only in the cathedral and diocese of Milan. How Gregory noted the music is uncertain; the early notation and rules of plain song were so complicated that it is said 10 years' study was necessary to acquire mastery of them. Among the plain song melodies used in the Roman church service of the Holy Week, those of the Lamentations of Jeremiah and of the Exultet, a portion of the Holy Saturday service, are by musicians de clared to be "beautiful beyond all description"; and the Exultet has been regarded as the finest example of plain song in existence.