The first or main stone placed at the corner of a building, traditionally important for its structure or ceremony. More generally, it means the most important part or foundation of an idea, system, or belief.
A compound of *corner* and *stone*, from the practice of setting a special stone at the corner of a building. This stone often carried dates or inscriptions and symbolized the building’s foundation.
From actual stones to abstract ideas, 'cornerstone' always points to what everything else rests on. People call things like trust, education, or free speech the 'cornerstone' of society to show how foundational they are. If you remove a real cornerstone, the building may shift; remove a social cornerstone, and society does too.
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