Calligram

At

the 

top

is a

cupola

then the

keystone

and then the crest

of the arch at the top of the

dome,  spreading,  elegantly  into  the

haunch of  the  arch - where  thrusting  forces 

begin to push out, fighting gravity by using gravity:

depending on the weight of all the masonry to force the

voussoirs together into a consolidated whole relying on the

weight above it, but with all that weight the walls begin to get

thicker toward the bottom, in order to prevent any cracks from

developing near the bottom of the dome the thickness acts as a 

restraining force, with more thick walls at the base total restraint.