The Arch is based on a fundamental principle used in constructing self-supporting structures that can span large distances and bear heavy loads. The 'Voissoir' arch principle, developed by the Romans, can be seen in many of their bridge and aqueduct structures still standing today. Building a 'Keystone Arch' requires the cooperation of several chilren. The blocks need to be positioned spatially witht the final piece at the top of the arch necessary to keep all other pieces in place; therein the name 'Keystone'. To build one is challenging, great fun and constructive learning.