At the convent of St. Gall originated the class of liturgical hymns called Sequences, which includes some of the finest examples of mediaeval hymnody. At a very early period it became the custom to sing the Alleluia of the Gradual to a florid chant, the final vowel being extended into an exceedingly elaborate flourish of notes. Notker Balbulus, a notable member of the St. Gall brotherhood in the ninth century, conceived the notion, under the suggestion of a visiting monk, of making a practical use of the long-winded final cadence of the Alleluia. He extended and modified these melodious passages and set words to them, thus constructing a brief form of prose hymn. His next step was to invent both notes and text, giving his chants a certain crude form by the occasional repetition of a melodic strain. He preserved a loose connection with the Alleluia by retaining the mode and the first few tones. These experiments found great favor in the eyes of the brethren of St. Gall; others followed Notker’s example, and the Sequence melodies were given honored places in the ritual on festal days and various solemn occasions. The custom spread; Pope Nicholas I. in 860 permitted the adoption of the new style of hymn into the liturgy. The early Sequences were in rhythmic prose, but in the hands of the ecclesiastical poets of the few centuries following they were written in rhymed verse. The Sequence was therefore distinguished from other Latin hymns only by its adoption into the office of the Mass as a regular member of the liturgy on certain festal days. The number increased to such large proportions that a sifting process was deemed necessary, and upon the occasion of the reform of the Missal through Pius V. after the Council of Trent only five were retained, _viz._, Victimae paschali, sung on Easter Sunday; Veni Sancte Spiritus, appointed for Whit-Sunday; Lauda Sion, for Corpus Christi; Stabat Mater dolorosa, for Friday of Passion Week; and Dies Irae, which forms a portion of the Mass for the Dead.