A tool for driving headed fasteners has an opening in the tool. The opening is defined by a wall of the tool. The wall has a plurality of flat planar surfaces. The planar surfaces are connected by a connection at the ends of each planar surface to define the opening. Each connection surface has a convex surface continuous with the planar surface. A concave surface is continuous with the convex surface. The convex and concave surfaces define arcs with the arcs having the same radius.