Alternatim

Alternatim refers to a technique of liturgical musical performance, especially in relationship to the Organ Mass. A specific part of the ordinary of the Mass (such as the Kyrie) would be divided into versets. Each verset would be performed antiphonally by two groups of singers, giving rise to polyphonic settings of half of the text. One of these groups may alternatively have consisted of a soloist, a group of instruments, or an organ. The missing even- or odd-numbered verses were supplied by plainchant or, perhaps more commonly (to judge by the organ masses of Hans Buchner), by improvisations on the organ. The verso became a particularly prevalent genre in Baroque Iberian organ music.