"A keystone (also known as capstone) 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 the stones into position, allowing the arch or vault to bear weight."

"final piece placed during construction"

"

" locks all the stones into position"

"Apply the Keystone"

"Find the Keystone"

"You have the keystone"