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1. A central cohesive source of support and stability
   "the keystone of campaign reform was the ban on soft money";
   - anchor, mainstay, backbone, linchpin, lynchpin
    
2. The central building block at the top of an arch or vault
   - key, headstone

Derived forms: keystones

Type of: building block, support

Part of: arch

Encyclopedia: Keystone, South Dakota