Mechanics of materials theory developed for axial deformation, torsional deformation, and bending can be used to compute the normal and shear stresses at a point in a physical object. Typically, these stresses are computed in reference to an arbitrarily defined coordinate system such as the x-y coordinate system.

The material comprising a structural member will fail based on the magnitude and the direction of the largest stress. Designers have no assurance that stresses computed in either the x or y directions are in fact the largest stresses at the point. It is possible that the largest stress magnitudes at a point occur at orientations other than the arbitrarily chosen x and y directions.