Considering the principal directions as the coordinate axes, a plane whose normal vector makes equal angles with each of the principal axes (i.e. having direction cosines equal to | 1 / 3 | {\displaystyle |1/{\sqrt {3}}|} ) is called an octahedral plane. There are a total of eight octahedral planes (Figure 6). The normal and shear components of the stress tensor on these planes are called octahedral normal stress σ o c t {\displaystyle \sigma _{\mathrm {oct} }} and octahedral shear stress τ o c t {\displaystyle \tau _{\mathrm {oct} }} , respectively.