Dictionary:

capstone

  (kăp'stōn')

1. The top stone of a structure or wall.
2. The crowning achievement or final stroke; the culmination or acme.

1. Any single stone in a coping.
2. A stone placed at the top of a stone arch.

The noun has 2 meanings:

Meaning #1: a final touch; a crowning achievement; a culmination
  Synonyms: finishing touch, copestone

Meaning #2: a stone that forms the top of wall or building
  Synonyms: copestone, coping stone, stretcher

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