“While the keystone is the principal locking stone of the Arch, the Copestone is the top, or cap of the entire wall. Therefore, the Royal Arch of Freemasonry, which has been classed as the 'the Copestone of Ancient Craft Masonry,' is known as the Capitular Rite.”
Frederick G. Speidel K.Y.G.C.H.
The York Rite of Freemasonry