key·stone /ˈkiˌston/
 拱心石,楔石,重點
 Key·stone n. Arch. The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorated with carving. See Illust. of Arch.
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 keystone
      n 1: 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"
           [syn: anchor, mainstay, backbone, linchpin, lynchpin]
      2: the central building block at the top of an arch or vault
         [syn: key, headstone]