A keystone is the wedge-shaped stone piece at the apex

Apex (geometry)

In geometry, an apex is the vertex which is in some sense the highest of the figure to which it belongs.*In an isosceles triangle, the apex is the vertex where the two sides of equal length meet, opposite the unequal third side....

 of a masonry vault

Vault (architecture)

A Vault is an architectural term for an arched form used to provide a space with a ceiling or roof. The parts of a vault exert lateral thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required...

 or arch

Arch

An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...

, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. This makes a keystone very important structurally. Although a masonry arch or vault cannot be self-supporting until the keystone is placed, the keystone experiences the least stress of any of the voussoir

Voussoir

A voussoir is a wedge-shaped element, typically a stone, used in building an arch or vault.Although each unit in an arch or vault is a voussoir, two units are of distinct functional importance: the keystone and the springer. The keystone is the center stone or masonry unit at the apex of an arch. A...

s, due to its position at the apex.