Moved progressively upwards as the dome rose,
they enUlld lliL diL LJlii iiuaajvl iibs vvuuld ulLimately converge at the
level of the fourth stone rinp. In order for these ribs to serve as guides for
the rest of the dome's vertical curvature, they were built first: that is, only
after several courses of the bricks for the ribs were laid did the masons
begin filling in the intermediate sections.