A keystone is the central, wedge-shaped stone at the summit of an arch, where the arch is at its most narrow point. It is typically the last stone to be placed during the construction of the arch and holds all the other stones in place.
A voussoir is one of the wedge-shaped stones that make up an arch, including the keystone. Together, the voussoirs distribute the weight of the arch evenly to the supporting foundations.
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