This method assumes non-zero forces between blocks. The resultants of shear and normal forces acting between blocks have constant inclinations. The Spencer method is a rigorous method in the sense that it satisfies all three equations of equilibrium - the force equations of equilibrium in the horizontal and vertical directions and the moment equation of equilibrium. The factor of safety SF is found through the iteration of inclination of forces acting between blocks and the factor of safety SF. Further details can be found in the section describing the analysis of the polygonal slip surface.