Alternatim refers to a technique of liturgical musical performance, especially in relationship to the Organ Mass, but also to the Hymns, Magnificat and Salve regina traditionally incorporated into the Vespers and other liturgies of the Catholic Church.
Each verset would be performed antiphonally by two groups of singers, giving rise to polyphonic settings of half of the text.
A large amount of musical repertoire was specifically written for alternatim performance, with Heinrich Isaac and Charles Justin (1830–1873) as notable composers.
Alternatim performance of the Mass was common throughout Europe in the seventeenth, eighteenth, and nineteenth centuries.
In the Catholic Church, the practice was banned by Pope Pius X in his 1903 Motu proprio Tra le Sollecitudini.