Everett R. Turnbull & Ray V. Denslow
The Keystone has been defined as the uppermost and last set stone of an arch which completes it and locks its members together.
This definition is illustrated by the following quotation from Bonar's Light and Truth:
"No man can estimate the extent of evil done by the loss of a simple truth. The Keystone of an arch is not always the largest stone. Figuratively the essential or fundamental element, as of a science or doctrine; the radical principle, as Christ, "The Keystone of the Church".
The Keystone has been identified with the Cornerstone, the Copestone, Capstone and the Foundation Stone, but these words are not identical, while any of these stones may be used as a Keystone in some situations, such use is not common. It may be well to define each of these words, that the distinctions as well as the similarities between them may be clear.
Capstone and Copestone are practically synonymous; both are defined as "the top stone of the wall" and their distinction, if any, is in their derivation. A Cap is the covering of the top of anything and a Cope is anything that arches overhead; thus a Capstone is a stone that furnishes a cap to a wall, and a Copestone is a stone arching the top of a wall. The Capstone or Copestowe of a building is the topmost stone of that building and is identical with the keystone, only when the principal arch of the building is at the top. Cornerstone is a stone uniting two walls at the corner of a building; especially one of those stones placed in the most prominent corner of the foundation. Figuratively, something regarded as fundamental or of primary importance as the "Magna Charter:' is the Cornerstone of English Liberty and obedience is the cornerstone of home training.
Thus it is in its symbolic meaning of being the most important stone that the Keystone may become the cornerstone, as it was in the Mark Master Degree.
Foundation Stone is one of the stones of the foundation of a building, or other structure, especially the Cornerstone.
Here again the similarity between the Keystone and the Foundation Stone is in their respective importance to the building. Symbolically, the most important element in a movement or organization or a proposition is interchangeably called its Keystone, Cornerstone, Copestone or its Foundation Stone. Masonically, there is a distinction between each of these stones, as well as a similarity.
Attention has been called to the fact that while the Copestone is the top stone of the wall or building, a Keystone is the top stone of the principal arch, but the Keystone has the added property of completing and locking the members of the arch together.
Thus the Keystone is peculiarly appropriate to Royal Arch Masonry. It has a very important place in Capitular Masonry. There are three degrees more or less built around it.