This double angle relation comes from the fact that the parametric equations for the Mohr circle are a function of 2 θ θ . It can also be seen that the planes A and B in the material element around P of Figure 5 are separated by an angle θ θ = 90 ∘ ∘ , which in the Mohr circle is represented by a 180 ∘ ∘ angle (double the angle).