Explanation of the Prayers and Ceremonies of Holy Mass, Dom Prosper Guéranger - Sequence

On certain Solemnities, there is added to the Alleluia or Tract, what is called the Sequence, (Sequentia).  It was added to the chant of the Mass long after the time of St. Gregory; the addition was made some time about the 9th century.  It received the name of Sequence, that is to say, sequel, because it originally consisted of certain words adapted to the notes which form a sequel to the word Alleluia, and which were called Sequentia, even before the introduction of the Sequence.

It is called, also, the Prose (Prosa,) because originally, it bore no resemblance either to the metrical hymns composed by ancient writers, nor to cadenced rhythms, which appeared later on.  It was a real piece of prose, which was sung in the manner we have described, as a way of putting words to the pneuma of the Alleluia.  By degrees, however, it partook of the character of a Hymn. - The Sequence thus added to the solemnity of the Liturgy; and, whilst it was being sung, the Bells were rung, as now, and the Organ was played.  There was a Sequence for every Feast, and, therefore, for the Sundays during Advent.  In the Roman Missal drawn up by order of St. Pius the Fifth, only four of the Sequences were retained.  These four are, the Victimae Paschali, which is the most ancient of all, and was followed as the model of the rest; the Veni Sancte Spiritus, the Lauda Sion, and the Dies irae.  Later on, there was added the Stabat Mater.  The Monastic Missal has also the Laeta dies, for the feast of St. Benedict; it is a composition of the 16th century.