I love the way that architectural terms work their way into general language (or sometimes the other way around). In architecture, when building a stone arch, one uses blocks called voussoirs. Each voussoir is precisely cut so that it presses against the surface of its neighboring blocks. The keystone is that wedge-shaped stone placed at the apex of an arch. It’s the final piece placed during construction and it is literally the key that locks all the other stones in place and allows the arch to bear weight. Curiously, it’s actually the stone that takes the least stress of any of the other voussoirs. In the 16th century, Mannerist architects liked to play with the position and size of the keystone, creating a feature by enlarging it or dropping it down slightly. Oh and if you’re wondering… Pennsylvania was called the Keystone State because of its position (both geographical and political) in the thirteen colonies when Pennsylvania’s vote for independence was the keystone vote.