Have you ever wondered why there is extra music at Mass sometimes right before the Alleluia and proclamation of the Gospel? This "extra" music is called a sequence. A sequence, in the context of the Mass, is a chant sung immediately prior to the Alleluia and the proclamation of the Gospel.
The Latin sequence originated around the ninth century in a much different form from the Gregorian sequences that are used today (Gregorian sequences in their current form date back to the eleventh century). Gregorian sequences still in use in our modern-day liturgies include the Easter sequence "Victimae paschali laudes" ("Christians, praise the Paschal Victim"), Pentecost sequence "Veni Sancte Spiritus" ("Come, Holy Spirit"), Corpus Christi sequence "Lauda Sion" ("Zion, praise your Savior"), Our Lady of Sorrows sequence "Stabat Mater" ("The grieving Mother stood"), Christmas sequence "Laetabundus" ("Faithful people"), and the Requiem Mass sequence "Dies irae" ("Day of wrath"), which was transferred in 1970 to the Liturgy of the Hours in the week preceding Advent.
