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
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Keystone
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 keystone
It is the keystone.