The top stone of a wall or structure; also used metaphorically to mean the finishing touch or crowning achievement of something.
Compound of cap (top) and stone; literally the stone that caps or tops a structure. Used in architecture since medieval times, extended to metaphorical use in the 19th century.
The capstone is the last stone a builder places, which is why it became the perfect metaphor for graduation ('capstone courses'), completion of careers, and final life achievements—it's literally the point where you stop building!
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