copestone

from WordNet (r) 3.0 (2006)

copestone
    n 1: a final touch; a crowning achievement; a culmination [syn:
         {finishing touch}, {capstone}, {copestone}]
    2: a stone that forms the top of wall or building [syn:
       {capstone}, {copestone}, {coping stone}, {stretcher}]

from The Collaborative International Dictionary of English v.0.48

Copestone \Cope"stone`\, n. (Arch.)
   A stone for coping. See {Coping}.
   [1913 Webster]