Keystone (architecture)
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A keystone is the wedge-shaped stone at the apex of a masonry arch or typically round-shaped one at the apex of a vault. In both cases it is the final piece placed during construction and locks all th
Dropped keystone at Colditz Castle
Keystone from the palazzo Borgazzi (Milan, Italy)
Keystone much enlarged for decorative effect, and carrying a coat of arms, Barcelona
The York Minster Chapter House rib-vault ceiling with central and peripheral keystones
Voussoir
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A voussoir is a wedge-shaped element, typically a stone, which is used in building an arch or vault.
Keystone (architecture)
…Although a masonry arch or vault cannot be self-supporting until the keystone is placed, the keystone experiences the least stress of any of the voussoirs, due to its position at the apex. Old keystones can decay due to weathering and vibration, a condition known as bald arch. In a rib-vaulted ceiling…
William Henry Playfair, University of Edinburgh: bevelled edges of each stone block emphasise the voussoirs, which have a curved base and together form a semi-circle at the top of each arch.
Joggled voussoir arch, located at the Mausoleum of Theodoric in Ravenna, Italy. (520 CE)
Keystone arch, Italian courtyard, 16th-century late Renaissance (Mannerism).
A single voussoir unit, made of marble, 6 5/8 x 14 1/2 in. (16.8 x 36.8 cm) 1130–40.