"keystones" in the noun sense
1. anchor, mainstay, keystone, 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"
2. keystone, key, headstone
the central building block at the top of an arch or vault
Source: WordNet® (An amazing lexical database of English)
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