The term 'keystone structure' is a reference to an ARCH, which is a curved structure that is used to span an opening and support the weight of a structure above it. An ARCH is composed of wedge-shaped blocks called voussoirs, which are held together by gravity and the outward pressure of the ARCH's curve. The keystone is the central voussoir at the top of the ARCH, which locks all the other stones into place and provides stability to the entire structure.