sentences:  Although Striggio left no performance directions, the most likely arrangement was for the singers to be positioned in a large semicircle, with the instrumentalists in its center, in view of the singers.The exact instruments used, and their number, is not known, but probably varied from performance to performance.An unusual feature of the mass is the existence of two partbooks for a "Bassus ad organum", a part which doubles the composite bass line of the entire composition.