What does the keystone do in an arch?

The keystone plays a crucial structural role in an arch by serving as the central stone that locks all the other stones of the arch in place. It is typically the highest stone in the arch, and its shape is designed to allow for an even distribution of weight down through the arch and into the supporting structures below. This key function prevents the arch from collapsing by maintaining the integrity of the structure, as the tension created by the other stones is directed downwards, stabilizing the entire arch.

While improving aesthetic appearance and acting as a decorative element are relevant characteristics of some keystones, they are secondary to its primary function of structural support. Additionally, insulation is not a role associated with the keystone in architectural terms, as insulation pertains to materials that help regulate temperature within a building, rather than the load-bearing qualities of an arch. Therefore, the keystone's essential contribution is its ability to hold the other stones securely in place, ensuring the stability and longevity of the arch.