Catenary
When you suspend a chain from two hooks
and let it hang naturally under its own weight,
the curve it describes is called a catenary.
Any hanging chain will naturally find this
equilibrium shape, in which the forces of
tension (coming from the hooks holding the
chain up) and the force of gravity pulling
downwards exactly balance. Something
beautiful happens when you turn a catenary
curve upside down. The inverted catenary will
now describe an arch — and it turns out that
it's the most stable shape an arch can have.
Gateway Arch is a catenary.