Keystone

The central and highest stone (voussoirs) in an arch. It cannot fall out of place because it is wedge-shaped, with the widest part of the wedge at the top. It is the last stone to be set in place during the construction of an arch. By pressing equally on either side it holds the arch together.An example:A carved keystone on the City Hall in Stockholm, Sweden. See grotesque.Also see crown and springing.