The cantus firmus is sung by the soprano. The second tenor begins alone with the cantus firmus. The first tenor begins alone with the cantus firmus. The sopranos sing the cantus firmus, the other voices interpret aspects of the narration. Examples of cantus firmus masses include "Corona Spinea" and "Gloria Tibi Trinitas". The mass is freely composed, not based upon a cantus firmus or parody. This is used as the cantus firmus of the mass setting. Several writers have used "cantus firmus" as a metaphor. The graph of the "Ursatz" is arrhythmic, as is a strict-counterpoint cantus firmus exercise. In the adjacent example in two parts, the cantus firmus is the lower part. The virelai is notated for two voices, cantus and tenor. As polyphonic music developed, a second voice above the "cantus" was termed the "contratenor". Several sub-projects have already been successfully completed, including the Liber Usualis and Cantus Ultimus. Throughout Ariosto's writing are narratorial comments dubbed by Dr. Daniel Javitch as "Cantus Interruptus". People at a cantus use special formulae, usually in Latin.