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1. The fundamental assumptions from which something is begun or developed or calculated or explained
   "the whole argument rested on a cornerstone of conjecture"
   - basis, base, foundation, fundament, groundwork
    
2. A stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremonies
    
3. A stone at the outer corner of two intersecting masonry walls

Derived forms: cornerstones

Type of: assumption, stone, supposal, supposition

Part of: building, edifice, explanation

Encyclopedia: Cornerstone Cornerstone, Pennsylvania