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My mistake.
I meant to say KEYSTONE, and the definition of KEYSTONE is exactly what I had in mind.
From Grammarist.com:
A keystone is the central stone placed at the top of an arch. The keystone is the apex of an arch, without it the arch would not stand. The keystone is placed last when constructing an arch, locking all the other stones into place. It allows the arch to bear its own weight. The word keystone is often used figuratively to mean the central idea of a philosophy, process, business proposition or principle upon which the entire philosophy, process, business proposition or principle stands. The term keystone has been in use since mid-1600s. The keystone is sometimes referred to as the capstone, but strictly speaking, the two terms have different definitions.
Agree keystone.