A keystone is the uppermost, central stone in a masonry or stone arch. This wedge-shaped stone is placed last and ‘locks’ the arch. Only once the keystone is in place can the arch support loads independently, and the centering (temporary support) be removed.
Characteristics of a keystone
| Characteristic | Description |
|---|---|
| Position | At the top, in the centre of the arch |
| Shape | Wedge-shaped (trapezoidal) |
| Function | Locks the arch and makes it self-supporting |
| Decoration | Often larger or decorated as an accent |
How it works within the arch
The keystone works together with the other arch stones (also called voussoirs):
- The weight presses on the keystone
- The keystone distributes the forces to both sides
- Via the voussoirs, the forces are directed to the springing points
- The piers or walls absorb the compressive forces
Decorative keystones
In many historic buildings, the keystone is more elaborately finished than the other voussoirs:
- Mascarons: sculpted faces or heads
- Coats of arms: family or civic heraldry
- Oversized keystone: extra large as a visual accent
- Moulded keystone: with mouldings or profiling
Tips for the DIYer
- When building an arch in brickwork: always work from both springing points towards the centre
- The keystone is placed last of all
- Use a centre (timber former) as support during construction
- Do not remove the centering until the mortar has fully cured
Related terms
- Arch: curved structural element above an opening
- Voussoirs: the wedge-shaped stones of the arch
- Springing: the starting point of the arch
- Centering: temporary timber support used when building an arch