Every stone fits and stays where it should in a well built dry stone wall. There is a maximizing of equilibrium. Unlike precarious 'stone balancing' where the stones barely touch and fall down within hours, the points of contact in a dry laid wall are many. The stones are all held in a stable balance. They lean on each other and give the wall permanent strength . Gravity is the glue, not cement, and yet long term 'balance' is attained.