anchor, mainstay, keystone, backbone, linchpin, lynchpin (noun)

a central cohesive source of support and stability

"faith is his anchor"; "the keystone of campaign reform was the ban on soft money"; "he is the linchpin of this firm"

keystone, key, headstone (noun)

the central building block at the top of an arch or vault

keystone (Noun)

The top stone of an arch.

keystone (Noun)

A native or resident of the American state of Pennsylvania.

Keystone

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. 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, as in the "church house" entrance portal at Colditz Castle. Numerous examples are found in the work of Sebastiano Serlio, a 16th-century Italian Mannerist architect.