What is a Keystone?
A keystone is the top block of an arch. It is an essential part of an arch because it keeps other stones from falling.  A keystone is shaped like a trapezoid, the widest part at the top, so it doesn't slide out of the arch.  Humans have built remarkable buildings with heavy ceilings on arches supported by a keystone.  Many architects and artisans have been known to sculpt family emblems, symbols and designs on keystones.
O que é uma Keystone?
A pedra-chave é o bloco superior de um arco. É uma parte essencial de um arco porque impede que outras pedras caia. Uma pedra-chave tem a forma de um trapezoide, a parte mais larga no topo, para que não escorregue do arco.