Orchestra inter grados imos quod diametron habuerit, eius sexta pars sumatur, et in cornibus, utrumque aditus eius mensurae perpendiculum interiores sedes praecidantur, et quae praecisio fuerit, ibi constituantur itinerum supercilia; ita enim satis altitudinem habebunt eorum confornicationes.
As to the orchestra, a sixth part is to be taken of its diameter between the lowest steps. On the wings at either side of the entrance, the inmost seats are to be cut back to a perpendicular height equal to that sixth. Whatever the amount of this cutting off is, fixes the spring of the arch over the passages. In this way their vaulting will have sufficient height.
final_alignments\Vitruvius_Architecture_Book_5.json