traditional arch bridges were built by first having a wood framing on which to lay all the stones in a solid arch. That wood framing is called falsework, and is necessary until the arch is complete and can stand on its own. Only then is the falsework taken away. Without it, no such bridge would be left standing. That temporary structure, even if no trace is left of it at the end, is nevertheless critical to getting a functional bridge.

A Bridge Over a River Never Crossed
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