keystone

Keystone is a native English compound: key (the most important element, that which unlocks or determines) plus stone. The architectural keystone is the central voussoir — the wedge-shaped block — at the crown of an arch. In arch construction, all the other voussoirs lean inward against each other, held in mutual compression, but they cannot achieve equilibrium until the keystone is set. The arch is built on temporary wooden centering (a curved wooden form called a centering or falsework) that holds the voussoirs in position during construction. When the keystone is set, it completes the arch, and the centering can be removed — the arch is now self-supporting, holding itself in compression through the wedge action of the keystone. Remove the keystone and the arch collapses. This structural fact gave the word its metaphorical meaning: anything on which the rest of a system depends.

The arch itself was one of the great discoveries of ancient engineering. The Mesopotamians used corbelled arches in the third millennium BCE, but the true arch — with wedge-shaped voussoirs in mutual compression — was developed by Etruscan and Roman builders to a level of mastery that remained unsurpassed until the Industrial Revolution. Roman arches and vaults permitted spans and spaces that post-and-lintel construction could not achieve: the Pantheon's concrete dome (c. 125 CE), with its interior diameter of 43.3 meters, stood as the largest enclosed space in the world for over 1,300 years. The keystone principle — mutual compression creating structural stability — was the engineering insight behind all of Roman vaulted construction, from sewers to amphitheaters to triumphal arches.

The medieval Gothic arch refined the Roman semicircular arch into the pointed form that is the signature of Gothic architecture. The pointed arch has the mechanical advantage of directing thrust more steeply downward than a semicircular arch, reducing the lateral thrust that must be resisted by the supporting walls or buttresses. Gothic keystones were often elaborately carved — at the intersection of vaulting ribs, they became the visual focal point of the vault, sometimes carved as foliage bosses, sometimes as figures, sometimes as coats of arms. The carved keystone is the point where structure and ornament most perfectly coincide: the most necessary block in the construction is also the most decorative.

The metaphorical keystone entered English usage in the seventeenth century and quickly established itself as one of the most useful structural metaphors in the language. 'Keystone of foreign policy,' 'keystone of the argument,' 'keystone species in an ecosystem' — all of these use the arch as a model for any system in which one element determines the stability of all the others. The Pennsylvania state nickname 'the Keystone State,' dating from the early nineteenth century, refers to Pennsylvania's central position among the original thirteen colonies — it was the geographic keystone that held the union together. The keystone metaphor works because the arch itself is such a vivid demonstration of the principle: one element, of no greater mass or material than its neighbors, determines by its position whether the whole stands or falls.