i 4 4 THE HISTORY OF MEDIEVAL EUROPE supported on the keystones of, and by pendentives between, four great round arches springing from four great piers, placed at the four corners of a square large enough to cir- cumscribe the circumference of the dome. Pendentives are triangular segments of masonry which carry the weight of the hemispherical dome down to the four piers and which thus make the transition from the circular space above to the larger square opening below. Two of the great arches GROUND-PLAN OF THE CHURCH OF ST. SOPHIA opened into half domes, beneath which, as beneath the great central dome, was open floor space. What is more, each half dome rested upon, and opened into, three smaller half domes or apses. The two great arches on the north and south sides of the dome, however, were filled in with sup- porting arches and columns, and beneath them were porti- coes running along either side of the main auditorium. If we include these porticoes beyond the columns at either side of the open floor space beneath the domes, we have an interior two hundred and thirty-five by two hundred
