If you are refering to bridge design, a capstone or keystone, is the center piece of the arch that locks the two ends of the bridge together. In an arch, each brick or stone is stacked so that it spans half an arch on each end. This would fall, except that the capstone is what connects the two ends. Its usually shaped as such, \ / , and allows the two arches to lean on eachother and thus support eachother and not fall. You might be refering to something different though.
Edit: Oh yeah, its also used in old arched doorways and cathedrals, same concept as in the bridge, only flatter.
Here is a picture of two arches one on top of the other, I have never seen this before, but you can clearly see the TWO capstones or keystones at the top of the arch, which I highlighted red for you. I found the pic on the net, it is of Hemyock Castle
Is this in reference to the architectural art or the college courses? As an incoming engineering freshman, I know a lot of schools have a capstone project in their senior year, where you actually have to complete a project. The goal of this is to demonstrate that you have actually learned the material and how to apply it. This help?
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