cornerstone

Noun

• a stone at the outer corner of two intersecting masonry walls

• a stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremonies

• the fundamental assumptions from which something is begun or developed or calculated or explained
  • the whole argument rested on a basis of conjecture
    

Related words

• Synonyms: 
  basis , base , foundation , fundament , groundwork