A ball joined to a first element, a hole through the ball and first element, a socket, having a spheroidal internal surface, joined to a second element, a hole through the socket and second element, said socket containing the ball in mating and nesting arrangement with the hole in the ball communicating with the hole in the socket, one of the ball and the socket containing at least one cam slot extending along a line of longitude, a cam follower having an axle joined to one of the ball and the socket at the equator and extending into the slot in the other one, and said ball and socket being rotatable relative to each other around only two of three orthogonal axes intersecting at a point in the radial center of the ball.