instruction: What is a Keystone?
response: 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.