In the field of design and architecture, we often use keystones. Here is a photo I took years ago in Rome of the Arch of Constantine in the Forum. It is a beautiful example of a keystone at the top of the archway. By definition, the keystone is the wedge-shaped stone that gets placed in the top of an arch to hold all the other stones in place so they don’t fall down.
The Romans were the first to figure out how to make an archway work with a keystone and now days you can find examples of keystones all around the world. Originally, they were flat on the face but over time they got embellished with lions, gargoyles, acanthus leaves and all sorts of decorative touches. Look around and you will see them in use over doorways, at the apex of a vaulted ceiling and over windows. They have become such a significant element in traditional design that they are even mimicked in cabinetry and mantle hoods and applied as decorative trim over doors and windows in modern home construction. Over the weekend, see how many you can find in your neighborhood. I think you will be surprised.