The parts of an arch.
  To see how forces follow an arch shape, try out the interactive arch model on this page.
Ah, the game is afoot Watson!  The arch comes into use.  In the diagram at left, our friend stands below a small arch that might be part of a water system or tunnel.  Note the shape of the stones and how they are arranged.  The weight of the stones (and anything they are supporting above) is transferred in compression around the arc of the arch to the thick walls each side.

But there was a drawback.  As we see at right, if you make the arch big enough, the weight of the materials will want to push outward on the walls and the arch may collapse.  The solution to arch thrust is on the next page.