- This category of chant was first defined by Isidore of Seville (c. 559–636): "Responsories are so called because a chorus responds in consonance to a soloist" (New Grove, 15: 759). Responsories were sung especially at matins, where they take on a particularly luxurious form known as the great responsory, but also at vespers and at mass in the Alleluia. The simplest form is in three parts: the respond sung by the choir, the psalm verse sung by the cantor, and the repetendum, the last segment of the original respond, sung by the choir.Reponsories are outnumbered in the repertory of Latin chant only by antiphons. The earliest source, the Hartker Antiphoner (c. 1000), contains 600 of them; the Worcester Antiphoner (13th century) almost 1,000. Responsories hold a proud place in the history of early polyphony, including settings in the Magnus Liber Organi (c. 1170). Some scholars believe that responsories were originally simple, perhaps sung by the congregation, in alternation with an entire psalm. As the trained choir took over the singing, the responds became musically elaborate, and the psalm was reduced to a single verse. In the new Roman Catholic liturgy of 1969, the "responsorial psalm" approximates the supposed ancient practice.
Historical dictionary of sacred music. Joseph P. Swain. 2006.