Curiously that so many people, especially poets and sculptors, chant the keystone as a unique element of an arch. Reality is a bit different, though, since all the stones in an arch are keystones — they have the identical function and carry the equivalent load.
Yet, people keep repeating the mistake: "Remove the keystone, and everything will collapse!" Sounds compelling, but if you remove any of the stones from an arch, construction will collapse in the same way.
Usually, the keystone is distinguished from others. It can be larger or decorated with an ornament or animal head, while, in fact, it can be done with the fourth stone from the right — nothing will change.
It's a story about how people tend to exalt the roles of their superiors... While, in fact, the boss isn't that different at all — the same muzzle as everybody else.