Alternatim refers to a technique of liturgical musical performance. A specific part of the ordinary of the Mass (such as the Kyrie

Kyrie

Kyrie, a transliteration of Greek κύριε , vocative case of κύριος , meaning "Lord", is the common name of an important prayer of Christian liturgy, which is also called the Kýrie, eléison ....

) 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

Pipe organ

The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...

. The missing even- or odd-numbered verses were supplied by plainchant or, perhaps more commonly (to judge by the organ masses of Hans Buchner

Hans Buchner

Hans Buchner was an important German organist and composer....

), by improvisation

Improvisation

Improvisation is the practice of acting, singing, talking and reacting, of making and creating, in the moment and in response to the stimulus of one's immediate environment and inner feelings. This can result in the invention of new thought patterns, new practices, new structures or symbols, and/or...

s on the organ.

A large amount of musical repertoire was specifically written for alternatim performance, with Heinrich Isaac

Heinrich Isaac

Heinrich Isaac was a Franco-Flemish Renaissance composer of south Netherlandish origin. He wrote masses, motets, songs , and instrumental music. A significant contemporary of Josquin des Prez, Isaac influenced the development of music in Germany...

one notable composer. Alternatim performance of the Mass was common throughout Europe in the seventeenth, eighteenth, and nineteenth centuries.