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You might think that all arches are made the same way. The fact is they are not all the same.
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. The keystone helped to distribute the weight down the side supporting blocks (voussoir blocks) of the columns. With this design, the keystone is the "key" to supporting the arch, because if you remove the stone, the arch would collapse.
Test and see what happens when you remove a keystone from the arch above by placing your mouse point on the arch.
How can you build the sides before inserting the keystone?
During construction, a temporary wooden frame supported the side voussoir blocks until the keystone was inserted. After the keystone is inserted, the wooden frame is removed.
Tracy Johns. (2010, January 07). What is a Keystone?. ASU - Ask A Biologist. Retrieved February 21, 2020 from https://askabiologist.asu.edu/what-keystone
Tracy Johns. "What is a Keystone?". ASU - Ask A Biologist. 07 January, 2010. https://askabiologist.asu.edu/what-keystone
Tracy Johns. "What is a Keystone?". ASU - Ask A Biologist. 07 Jan 2010. ASU - Ask A Biologist, Web. 21 Feb 2020. https://askabiologist.asu.edu/what-keystone
Roman arch with a prominent keystone. Image by Sara Fuoco via Wikimedia Commons.