Gregorian chant, also called plain-song, is a musical form from the early Catholic Church in which all voices sing one part in unison. The music has no feeling of tempo, and most of the motion of the notes is step-wise. Most Gregorian chants are part of two liturgical rites – the Mass and the Offices. Origins traditionally are ascribed to the period of Pope Gregory I (590–604).
Distinguishing features of Gregorian chant are (1) its recitative-like character, as oppose. to what was styled musica mensuruta – i. e. barred music, with a marked and regular rhythm, which was the essential point of ancient Greek music, and more or less of nearly all modern music; (2) the modes, or scales, in which it is written, which are more numerous and varied than the modern major and minor; and (3) its being (originally) sung in unison, though much of it is susceptible of treatment in harmony, and is now frequently heard in this way.
It used to be stated also that the notes in Gregorian were all of equal length, but this view is now generally repudiated. It embraces music for all parts of the Roman services, from the Accents (nearly in monotone) proper to the various readings to the more elaborate melodies of the antiphons and hymns, and the various parts of the mass. The best known and most ancient of all is the music of the eight Tones sung to the Psalms, commonly called the Gregorian Tones. As to the origin of these many different views prevail, some ascribing them to a Greek, some to a Hebrew source, others to the early Christians; there seems some probability, though there is no direct evidence, that they were actually derived from the music of the temple service.
As at first plain-song was handed down orally only, and the early systems of notation were very defective, it is impossible to determine how far it may have been corrupted. It was first reduced to system by St Ambrose (d. 397) but much more extensively by St Gregory the Great, towards the end of the 6th century. There have been large additions since. How he noted the music is uncertain; the early notation and rules of plain-song were so complicated that it is said ten years' study were necessary to acquire a mastery of them. Local varieties of the proper melodies gradually sprang up, almost every diocese having an office-book peculiar to itself – e.g. the antiphonary and gradual of Sarum, said to be one o the purest.
The earliest known existing record of plain-song is the Antiphonarium, or rather Gradual, in the library of the monastery of St Gall in Switzerland, probably of the 9th or 10th century. Various directories have been published, notably that begun by Palestrina and finished by Guidetti ; the latest, issued under sanction of the pope, is the great series published at Ratisbon by Pustet, beginning in 1871 with the Gradual. The music is still always printed in the old square notes on a stave of four lines.
At the Reformation, Gregorian music was adapted to the new vernacular services of the English Church by John Marbeck, who published in 1550 The Book of Common Prayer noted; and his arrangement is still in use in cathedral services. Anglican Chants are modelled on the Gregorian psalm tones.
The modes, or scales, of plain-song require some explanation. Their variety has been acknowledged as affording greater resource of expression than our major and minor modes and music has been written in them by great modern composers – e.g. the "Hymn in the Lydian Mode" in Beethoven's Quartet, op. 132. They were derived from, though it is not certain that they were identical with, the Greek diatonic scales, after which they have been named. The principle of their formation is that each of the seven natural sounds of the diatonic scale forms the keynote, or "final" of a mode, which embraced that note and the seven above it. (The melodies rarely exceeded an octave, and no flats or sharps are found except an occasional B flat.) This would give us seven modes; but to each of these is attached another, in which the melody, while having the same final or keynote, instead of ascending to the octave above, ranges from the fourth below it to the fifth above. The former are called the authentic modes, the latter plagal. The difference of the modes, and the effect of the melodies in them, is owing to the various positions of the two semitones in the scales.
The difference between an authentic and a plagal melody may be illustrated from two psalm-tunes – Newton or New London, and the Old. Hundredth, in the first of which the melody lies between the keynote and its octave, but in the other between the fourth below and the fifth above the keynote. But while the whole fourteen modes are enumerated, for the sake of completeness in theory, two of them are universally rejected in practice as defective – the two having B as their keynote. The modes are, then, arranged in pairs as follows. The 1st or Dorian (authentic) embraces the notes from D on the middle line of the bass stave to the D above, and has its keynote on D; the 2d or Hypo-dorian (plagal) has the same keynote, but its compass is from the A below to the A above it ; the 3d or Phrygian (authentic) and its corresponding plagal mode, the 4th or Hypo-phrygian, have similarly their keynotes on the E of the third space of the bass stave; the 5th or Lydian and 6th or Hypo-lydian have F for final; the 7th or Mixolydian and 8th or Hypo-mixolydian have G; the 9th or Aeolian and 10th or Hypo-molian end on A ; then come the rejected modes on B, styled the Mixo-locrian and Hypo-mixolocrian; then the Ionian or Iastian and Hypo-ionian or Hypo-iastian on C, numbered variously as 11th and 12th, or 13th and 14th, according to the rejection or inclusion of the two preceding. The Ionian is the modern major mode.
St Ambrose's arrangement of the melodies was said to have been confined to the 1st, 3rd, 5th, and 7th modes (authentic); while the relative plagal modes, 2nd, 4th, 6th, and 8th, were added by St Gregory. In these are written the correspondingly numbered eight psalm tones; the Peregrine Tone, used only for the psalm In exitu Israel, is in the 9th mode. The other modes were finally added in the 8th century under Charlemagne. Each mode has its reciting note, or Dominant – not to be confused with the modern term in harmony.
Various examples of plain-song hymn melodies will be found in Hymns Ancient and Modern – e.g. No. 14, the vesper hymn of St Ambrose, " 0 Lux beata Trinitas"; and No. 96, the hymn of Fortunatus, "Vexilla Regis prodeunt", which may be studied in a different treatment by Gounod in the "March to Calvary" in his Redemption.