Sethus Calvisius (1556–1615), one of the very small number of specialists in the improvised vocal fugue, provided a discussion of the practice in his Melopoiia (1592), illustrated with 21 notated examples of fugæ extemporaneæ—tricinia, or two-part canons, over a cantus firmus.
These pieces were improvised as a third voice sang the cantus firmus, with the two improvising voices entering a minim or semibreve apart; the first of the two singers was effectively the composer. Analysis of Calvisius’s works shows that his mastery of the technique was complete, and he was capable of creating canonic improvisations of surprising originality.
This according to “Harmonia fvgata extemporanea: Fugenimprovisation nach Calvisius und den Italienern” by Olivier Trachier, an essay included in Tempus musicae–tempus mundi: Untersuchungen zu Seth Calvisius (Hildesheim: Georg Olms 2008, pp. 77–102). Below, the Dresdner Kreuzchor performs Calvisius’s Freut euch und jubilieret.