What is a Keystone?

What is a keystone? You might think that all arches are made the same way. The fact is they are not all the same. Typical Roman arch with keystone. It is true that Egyptian, Babylonian, Greek and Assyrian civilizations used arches for underground structures such as drains and vaults. However, it was the Roman civilization (1000 B.C.E. - 500 C.E.) that first began using a keystone (also called a capstone) in their arches. The keystone is the topmost stone in the arch. The one in the illustration on the right is exaggerated in size from what a normal keystone would be.

Keystone species are species that are more important than others in an ecosystem. This means that the loss of a keystone can lean to drastic changes

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Keystone (architecture) - Wikipedia

The Stone Arch Bridge

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KEYSTONE BICYCLE CO.