Beneath the Pendentive Bay                             -      N      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      S      -

The alternating compound pendentives to your far east and west soar into
the air, branching into a rib vault above you.  These vaults you see are
diagonal and transverse ribs, which compose a structural skeleton that
partially supports the fairly massive body of the building.  Other elements
you notice as you slowly proceed are the four large buttresses to your
north, supporting the impressive central dome that rests above them.  You
are motivated to continue north, to the tetrarch chamber.