In the physical-space sign convention, positive shear stresses act on positive faces of the material element in the positive direction of an axis. Also, positive shear stresses act on negative faces of the material element in the negative direction of an axis. A positive face has its normal vector in the positive direction of an axis, and a negative face has its normal vector in the negative direction of an axis. For example, the shear stresses τ τ x y and τ τ y x are positive because they act on positive faces, and they act as well in the positive direction of the y -axis and the x -axis, respectively (Figure 3). Similarly, the respective opposite shear stresses τ τ x y and τ τ y x acting in the negative faces have a negative sign because they act in the negative direction of the x -axis and y -axis, respectively.