from "Empires Ascendant" Time frame 400 BC - AD 200 Time Life Books To support the tremendous weight of the arches, it was necessary to provide a way of transmitting the force to massive piers to the foundation of the arch. The Romans achieved this feat through the use of the Keystone block. The force was directed down onto the top of the keystone. Because of its shape the force was translated to the voussoir blocks of the arch which in turn translated the force through the impost to the piers. During construction, the voussoir's were supported by a temporary wooden frame until the keystone was inserted.

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