the.com/cornerstone

means The foundational fact, person, or principle that everything else is built on and depends on for support.

from From Old English corner + stone, but the real weight comes from ancient building practice: the cornerstone was the first stone set, chosen with care because every other wall's alignment was measured from it. The Bible gives the word its metaphorical afterlife, calling a keystone figure the 'cornerstone' the builders rejected — and from there it walked into everyday speech as shorthand for anything indispensable the rest of the structure leans on.