Keystone

A keystone is the wedge-shaped stone placed at the very top of an arch or vault. The voussoirs rising from either side cannot stand on their own until it is in place, so during construction they rest on a timber centring. Once the keystone goes in, the arch is closed and begins to carry itself.

How the load travels

An arch passes the load above it down through its stones as compression, out to either side. The keystone is the top link in that chain of compression: it squeezes the voussoirs on both sides together and helps balance the load in each direction. Its wedge shape means that the more it tries to slip down, the harder it presses on its neighbours, tightening the whole system. Every stone in an arch matters, but the keystone is also where the geometry of the arch closes.

• Voussoirs: the wedge-shaped stones along the curve
• Springers: the starting stones at each end
• Keystone: the closing stone at the top

The keystone is often a little larger than the other voussoirs or projects slightly from the face. In some buildings it carries an inscription, relief or ornament.

In survey and restoration

A damage assessment checks whether the keystone has slipped downward. Such a slip can mean the arch is spreading, in other words that its supports are not resisting the thrust. Record the keystone’s size and level separately in survey. An arch modeled stone by stone in 3D makes it easier to identify each stone that needs replacing.