In the central dome we distinguish two changes of form.[br][list][*]A first tier changes the basic square into a regular octagon and and an octagonal drum sustains an octagonal dome.[br][/*][*]Ribs in this octagonal dome connecting the midpoints of the base, mark an octagonal as base for a second octagonal dome.[/*][/list]

In the applet below you can see how such a configuration looks like in 3D.

Looking at the octagon, and thinking of drawing geometric patterns we recognise it as the drawing of a 8.3 star within an octagon. This scheme with a second octagonal dome is used in the 12th century Iglesia del Santo Sepulcro (Holy Grave church) in Torres del Rio (Navarra - Spain).