It is shown that a system of forces consisting of three couples without moment introduces a cone of the second degree as the surface separating the compressions and dilatations as recorded at extended distances. This cone is the generalization of the pair of planes associated with a single couple with moment or with two couples without moment. The corresponding surfaces for SH and for SV are also given. The projection of these surfaces on the central plane is examined and some of the geometrical properties noted. In particular, it is seen that the projected S direction passes through a fixed point if two of the couples are equal. An example of the separation of the compressions and dilatations by a hyperbola is given.