cornerstone

Noun (3)

1. the fundamental assumptions from which something is begun or developed or calculated or explained“the whole argument rested on a basis of conjecture”Synonyms: basisbasefoundationfundamentgroundworkType of assumption, suppositionKinds meat and potatoesPart of explanation
2. a stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremoniesType of stonePart of building, edifice
3. a stone at the outer corner of two intersecting masonry wallsType of stonePart of building, edifice