cornerstone (n.)

also corner-stone, late 13c., "stone which lies at the corner of two walls and unites them" (often the starting point of a building), hence, figuratively, "that on which anything is founded;" from corner (n.) + stone (n.). The figurative use is biblical (Isaiah xxvii.16, Job xxxviii.6, Ephesians ii.20), rendering Latin lapis angularis.

In U.S. history, Alexander H. Stephens's Cornerstone speech explaining the new Confederate constitution was given at Savannah, Georgia, March 21, 1861. The image is older in U.S. political discourse and originally referred to the federal union.

I endorse without reserve the much abused sentiment of Governor M'Duffie, that "Slavery is the corner-stone of our republican edifice;" while I repudiate, as ridiculously absurd, that much lauded but nowhere accredited dogma of Mr. Jefferson, that "all men are born equal." No society has ever yet existed, and I have already incidentally quoted the highest authority to show that none ever will exist, without a natural variety of classes. [James H. Hammond, "Letter to an English Abolitionist" 1845]