Alternatim

Alternatim is a performance form of liturgical music. A specific part of the Mass Ordinary ( Kyrie, for example ) is divided into verses. Each verse is only sung antiphon of two singing groups and then in the second half in a polyphonic writing. One of the two groups may either consist of solo singers, an instrumental ensemble or an organ. For the lack of even or odd verses a simple chorale was sung or when an improvisation on the organ.

A large amount of music was composed in the alternatim performance form, for example by Heinrich Isaac. This form of measuring performance was spread throughout Europe from the 17th to the 19th century.