What is the top stone of a catenary arch?

The top stone of a catenary arch is known as the keystone. It is the central stone at the apex of the arch and is crucial for its structural integrity, as it locks all the other stones in place. The keystone helps distribute weight evenly across the arch, allowing it to effectively bear loads and maintain its shape. In a catenary arch, which resembles a hanging chain, the keystone ensures stability by counteracting the forces acting on the arch.