A base for supporting a load defines a first articular concave spherical surface segment forming a first socket about an axis extending through the center of curvature of the surface segment. The socket is nested with a first mating articular convex spherical surface segment forming a first ball segment...http://www.google.co.uk/patents/US4473206?utm_source=gb-gplus-sharePatent US4473206 - Articulated structure for supporting loads
