The crossword solution for the term 'Spot for a Keystone' is ARCH, which is a four-letter word. An ARCH is a curved structure that is used to span an open space and support the weight above it. It is often used in architecture as a decorative feature or to support a bridge or doorway. The keystone is the central stone at the top of an ARCH, which holds the other stones in place and gives the ARCH its strength. Therefore, the spot for a keystone is an ARCH.