The central wedge-shaped member of a masonry arch; also used as a decorative element on arches in wood structures
the most important part, from the idea of the central stone in an arch that keeps the other stones in position - without it, the whole arch would fall down
the symmetrically shaped, wedge-like stone located in a head ring course at the crown of the arch; the final stone placed, thereby closing the arch
A keystone of a policy, system, or process is an important part of it, which is the basis for later developments. Keeping inflation low is the keystone of their economic policy