Question

What do you mean by "keystone"?

Answer 1

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Keystone :-

The keystone is the most important stone, and that's why this word is also used figuratively to mean the most important part of anything. A stone arch or vault gains its stability from the placement of the keystone, which is often the last one placed.

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Answer 2

Explanation:

a central stone at the summit of an arch, locking the whole together.

                                          ( or )

the central principle or part of a policy, system, etc., on which all else depends.

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