Headstone

The cornerstone is the last stone that is placed to complete the arch. This uniquely shaped stone is an engineering marvel that is critical to supporting the arch and maintaining its strength.
The Keystone symbol is absent from the Lodge symbolism, but appears in the degrees of the Chapter. He figures in the York Rite of Masonic degrees in Freemasonry in the unfolding of the symbolic legend of Hiram, the builder of the Keystone.