Everyone knows what a keystone does, and what they know is slightly wrong. The stone at the top of an arch holds no more than its neighbours; the thrust grows on the way down, so if anything it carries the least, and take out any other stone and the arch falls just the same. It gets the credit because it goes in last, and the arch becomes an arch at that moment. What held everything up until then was a timber frame called the centering, built underneath to bear every stone for as long as the gap stayed open. Then it was knocked out and carted off to the next job. So the word we keep for the one indispensable piece belongs to whichever piece arrived at the end, and the thing that took the whole weight left nothing behind but the curve.