cornerstone

noun3

1. •
   A stone forming the base at the corner of a building.

   • Clutch it like a cornerstone Otherwise it all comes down Justify denials and Grip 'em to the lonesome end — 2001
2. •
   (figurative) That which is prominent, fundamental, noteworthy, or central.

   • Exceptional service is the cornerstone of the hospitality industry.
   • That is the cornerstone of any meaningful debate about budgets and projects, regulations and policies.
3. •
   Such a stone used ceremonially, often inscribed with the architect's and owner's names, dates and other details.

   • The cornerstone on the Flatiron Building is set on the Fifth Avenue facade.