Consider a point formula_12 in a continuum under a state of plane stress, or plane strain, with stress components formula_13 and all other stress components equal to zero (Figure 8.1). From static equilibrium of an infinitesimal material element at formula_12 (Figure 8.2), the normal stress formula_15 and the shear stress formula_16 on any plane perpendicular to the formula_17-formula_18 plane passing through formula_12 with a unit vector formula_20 making an angle of formula_21 with the horizontal, i.e. formula_22 is the direction cosine in the formula_17 direction, is given by: