cor·ner·stone 
/ˈkoɚnɚˌstoʊn/ noun
plural cor·ner·stones /ˈkoɚnɚˌstoʊn/ noun [count] 1 : a stone that forms part of a corner in the outside wall of a building and that often shows the date when the building was built
▪ Officials held a ceremony to lay the cornerstone for a new library. 2 : something of basic importance
▪ Trust is the cornerstone of their relationship.
▪ Officials held a ceremony to lay the cornerstone for a new library.
▪ Trust is the cornerstone of their relationship.
