The keystone, symbol of the Holy Vault. It is The stone placed in the center of an arch which preserves the others in their places, and secures firmness and stability to the arch. As it was formerly the custom of Operative Masons to place a peculiar mark on each stone of a building to designate the workman by whom it had been adjusted, so the Keystone was most likely to receive the most prominent mark, that of the Superintendent of the structure.
