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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
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Edication
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.
keystone
keystone
July 24, 1965
The highest point of an arch is called the "keystone." It is the central stone at the apex of the arch that locks all the other stones in place, allowing the structure to bear weight and maintain its shape. The keystone is crucial for the stability of the arch, distributing the forces evenly down to the supports.
The keystone