From BiblePortal Wikipedia

Sequences [1]

In chanting the Graduale in the Mass it was customary to prolong the last syllables of the Hallelujah through a succession of notes without words, which were termed sequences, when considered in their combination, and jubila orjubilationes with reference to their character. They were intended to indicate that feeling had reached a point at which it was too strong for expression. The difficulty of retaining a long series of notes in the memory led to efforts for devising mnemonic helps, which eventually resulted in the adoption of suitable rhythmical language in Latin prose to fit the music Notker Balbulus (q.v.; died 912), a monk of St. Gall, being especially distinguished in accomplishing this kind of work. The idea was suggested to him by some verses which were modulated or fitted to the series of tones in an antiphonarium belonging to a fugitive priest of Gimedia. He attempted to improve on them, and with such success that his teacher, Marcellus, a Scotchman, had his verses collected and sung by his pupils; and also persuaded Notker to dedicate his work to some prominent personage and give it to the world. Notker thus became the originator of an edifying element of worship, which was approved by the popes and speedily introduced into wider circles; and as he not only used the succession of tones already current the mettensis major and minor, the Romana and Amoena but also composed new series of notes, he became the creator of an elevating, melodious choir music which was inserted in the Mass. Each piece was divided into several parts and provided with an appropriate conclusion; and, in like manner, the text, which was everywhere adapted to the melody, consisted of a number of shorter or longer sections. A poetic character was thus naturally given to the text, and such compositions were consequently called "hymns" a term that is not misplaced when applied to those written by Notker. They were hymns of praise in which the leading features of a festival, the faithful support of the Almighty God, the Redeemer's merits, the dignity of the Blessed Virgin, etc., are fervently presented; while in their intent they were a continuation of the Hallelujah in the Gradual, though they might also be separately employed.

These sequences were introduced into use in Germany, England, France, and other countries. Notker's works became the type, and imitations in great number followed, until they were employed to edify the people at every festival; and more than one hundred were contained in the mass books. The revised Roman Missal contains but five viz. one to the Paschal lamb, intended for Easter; one for Pentecost (Veni Sancte Spiritus); one for Corpus Christi Day (Lauda, Sion, Salvatorem, by Thomas Aquinas); one intended to glorify the Mater Dolorosa (the celebrated Stabat Mater by Jacoponus); and one for use in masses pro defunctis, Thomas de Celano's judgment hymn Dies Iroe. The last two are most unlike the early sequences, as the Hallelujah could not be chanted with them; but they are at bottom jubilee hymns like the others.