#1
keystone n. [
anchor, mainstay, backbone, linchpin, lynchpin] 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"
. wn #2
keystone n. [
key, headstone] the central building block at the top of an arch or vault
. wn