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keystone
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A wedge-shaped stone placed at the apex of a masonry arch is called a keystone. During construction of the arch, the keystone is the final piece to be placed.
A brick or wedge-shaped stone forming an arch is called a "voussoir." These stones are positioned and interlocked to create the curve of the arch and distribute the weight evenly downwards and outwards to the supporting columns or walls.
The keystone
A keystone. PA was named the Keystone state as it was in the middle of the 13 colonies, much like a keystone in an arch or doorway. It's the stone at the top, middle of the arch which holds the arch in place. If the keystone is removed, the arch topples.
To hold the arch in place
Edication
The key stone is the stone placed at the apex of the arch, and prevents the two sides of the arch falling in and collapsing.
Keystone
keystone