A stone arch is made of wedge-shaped blocks called voussoirs (say “voo-SWAHR”). They are laid on a temporary wooden frame called the centering (or falsework), which holds each stone in the right place. The last stone, at the crown, is the keystone. Only when it is in place, and the centering is removed, does the arch take its load. Before that moment, the stones are just a pile that would fall if the frame vanished.

The keystone has a famous reputation, but it is not special in the way people think. Every voussoir carries the thrust. The keystone closes the ring and lets the compression start working, but it does not hold the arch up by itself. In many arches the keystone is actually among the least-loaded stones.