Although purely decorative, keystones are a great way to accent your windows and doors. Keystones should be initially placed when installing the rest of the molding and window trim, however - a center can be carefully cut from existing moldings to provide a place for keystones.
A keystone is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. This makes a keystone very important structurally. Although a masonry arch or vault cannot be self-supporting until the keystone is placed, the keystone experiences the least stress of any of the voussoirs, due to its position at the apex. Old keystones can decay due to vibration, a condition known as bald arch.
In a rib-vaulted ceiling, keystones may mark the intersections of two or more arched ribs. For aesthetic purposes, the keystone is sometimes larger than the other voussoirs, or embellished with a boss. Mannerist architects of the 16th century often designed arches with enlarged and slightly dropped keystones. Numerous examples are found in the work of Sebastiano Serlio, a 16th century Italian Mannerist architect.