keystone
US /ˈkiː.stoʊn/
UK /ˈkiː.stoʊn/
1.
a central stone at the summit of an arch, locking the whole together
:
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The architect carefully placed the keystone to complete the arch.
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Without the keystone, the entire structure would collapse.
2.
the central principle or part of a policy, system, etc., on which all others depend
:
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Education is the keystone of a prosperous society.
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The new policy is the keystone of their economic reform.