Cornerstone

[kawr-ner-stohn] /ˈkɔr nərˌstoʊn/

noun
1.
a uniting two masonry walls at an intersection.
2.
a representing the nominal starting place in the construction of a monumental building, usually carved with the date and laid with appropriate ceremonies.
3.
something that is essential, indispensable, or basic:
The cornerstone of democratic government is a free press.
4.
the chief foundation on which something is constructed or developed:
The cornerstone of his argument was that all people are created equal.
/ˈkɔːnəˌstəʊn/
noun
1.
a stone at the corner of a wall, uniting two intersecting walls; quoin
2.
a stone placed at the corner of a building during a ceremony to mark the start of construction
3.
a person or thing of prime importance; basis: the cornerstone of the whole argument
n.

late 13c., from corner (n.) + stone (n.). The figurative use is from early 14c.

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]