ALTERNATIM
Alternatim: translation
A specific kind of antiphony, by which traditional chant alternates with newly composed polyphony, almost always using the traditional response as a cantus firmus. In vocal music alternatim is usually practiced for the verses of a psalm or canticle. In an organ mass, alternatim is applied to the ordinary and proper prayers of the liturgy, with organ versets substituting for the chant at certain traditional points.