cornerstone

cornerstone (http://definr.com/cornerstone)

     n 1: the fundamental assumptions underlying an explanation; "the
          whole argument rested on a basis of conjecture" [syn: 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