In the context of liturgy, the word ‘sequence’ (from the Latin sequor, ‘to follow’) refers to a hymn that follows the gospel acclamation and thus prolongs the gospel procession on important feast days.
It started with the practice of the cantor extending the final ‘a’ of the gospel Alleluia on important celebrations. Gradually, words were added to the melody and by about the year 1000, the texts had been organised into extended rhyming verse. The words focussed on the particular feast or mystery being celebrated. These poems were always sung as hymns, never recited.
Eventually about 5 000 sequences came to be used in the liturgy. One of the most famous was the Dies Irae (‘Day of Wrath’) of the Requiem Mass.
Sequences now occur rarely in the liturgy. Pentecost Sunday is one of the two celebrations during the year that has an obligatory sequence: Easter is the other. There is an optional sequence for the feast of the Body and Blood of Christ.
Some of the traditional sequences survive as hymn texts, such as Stabat Mater at the Stations of the Cross and the Te Deum Laudeamus (‘Holy God, We Praise Thy Name’) as a hymn of praise on Trinity Sunday.
Although the sequence began as an extension of the Alleluia, it is now often read or sung after the second reading and before the Gospel acclamation, in contradiction of its intent of highlighting the acclamation and lengthening the Gospel procession.
The Tridentine missal prescribed the sequence to be sung after the gradual and the Alleluia and verse but before the Alleluia was repeated. In other words, it was an embellishment of the Gospel acclamation.
In reality, however, sequences were considered to be meditative hymns to which the assembly listened while seated, rather than as a preparation for the Gospel to be sung by all while standing.
When sung after the Alleluia as intended, the sequence serves as a processional hymn for an elaborate Gospel procession on two of the most important days of the Church year.