cor·ner·stone / kôr′nər-stōn′/, noun:
1. a stone uniting two walls which forms a corner of a building or structure
“The stone that the builder refused, will always be the head cornerstone.” Bob Marley, 1945 – 1981
2. an important quality or feature on which a particular idea or principle depends or is based upon. “Self-respect is the cornerstone of all virtue.” John Herschel, 1792 – 1871 “Education is the cornerstone of our communities and our country.” Bill Frist, 1952 -