The central stone in an arch that locks the arch in place is called what?

• A. Abutment
• B. Deck
• C. Keystone
• D. Spandrel

Answer: (C)

In an arch, stability comes from how the stones press against one another in compression. The wedge-shaped pieces, or voussoirs, transfer the load along the curve down to the supports. The stone at the crown—the central top stone—locks all the other voussoirs in place. This piece is the keystone, and once it is set, it completes the arch and converts vertical forces into compression that holds everything together and prevents the arch from spreading outward under load.

Abutments are the supports at the ends of the arch that resist the outward thrust, the deck is the roadway surface on top, and the spandrel is the space between the arch and the outer boundary structure.