In the passage quoted here from Monteverdi's madrigal [Cruda amarilli, mm.9-19 and 24-30], one sees a tonality determined by the characteristic of the accord parfait on the tonic, by the sixth chord assigned to the third and seventh degrees, by the optional choice of the accord parfait or the sixth chord on the sixth degree, and finally, by the accord parfait, and above all, by the unprepared seventh chord (with major third) on the dominant.

Joseph Fétis, (1844). Traité complet de la théorie et de la pratique de l'harmonie contenant la doctrine de la science et de l'art, 2d ed., p. 166. Brussels and Paris.