That's an interesting factoid I've never thought about before. Here's my guess why it's true. In a hanging chain, the sum of the forces pulling down (due to g

That's an interesting factoid I've never thought about before. Here's my guess why it's true. In a hanging chain, the sum of the forces pulling down (due to gravity/load) and away from the end (due to tension) on a given link must point in a direction exactly opposite the angle of the next chain link toward the end. Else that chain link would rotate until the above is true. In a stone arch, the sum of the forces pushing down (due to gravity/load) and toward the base (due to compression) on a given stone must point in a direction exactly equal to the next stone toward the base. Else that stone would rotate and the arch would break.