Archaic architectural term for a keystone, the central stone at the crown of an arch. Compare with the more common 要石 (かなめいし).
See also: 要石
In old documents, the stone at the top of an arch is sometimes called 剣石.
The term is composed of 剣 (sword) and 石 (stone), likely referring to the wedge-like shape of a keystone. The exact historical derivation is uncertain.