Keystone
KEYSTONE, the central voussoir of an arch (q.v.). The Etruscans and the Romans emphasized its importance by decorating it with figures and busts, and, in their triumphal arches, projected it forward and utilized it as an additional support to the architrave above. Throughout the Italian period it forms an important element in the design, and serves to connect the arch with the horizontal mouldings running above it. In Gothic architecture there is no keystone, but the junction of pointed ribs at their summit is sometimes decorated with a boss to mask the intersection.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
