The keystone is a machined stone (or "hooked" or "contag") to fulfill structural functions, placed at the top of an arc or once; It closes, with its wedge shape, the series of other constructive elements arranged side by side and is therefore indispensable element for downloading the straight weight from the bow on the lateral pillars.
The Etruscans were the inventors of the keystone as the first people of the Mediterranean Sea who introduced the arc in the constructions. From them the Romans learned the technique, who ably exploited in works such as the Colosseum and aqueducts. Previously the keystone had already appeared in Mesopotamia.
In Roman architecture, the key of the vault often has a façade, in particular on the triumphal arches, a decoration, more protruding than that of the rest of the blocks that make up the arc (generally shaped like curvilinear architrave, or archived). This is a large S-shaped shelf, arranged vertically, on whose face there are figurative representations, often of divinity.
#Keystone
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