What is the “cathedra”? And what is the “apse”?

Behind the altar lies the “cathedra,” where the bishop was seated on his throne surrounded by the priests who performed their worship and preaching; hence the use of the term presbyterum¹²⁰⁷ for this part of the church. Above the cathedra lies the “apse” which resembles God’s bosom that is wide open to the whole world through the altar and the priestly ministry.