The orientation of the planes where the maximum and minimum principal stresses act, also known as principal planes, can be determined by measuring in the Mohr circle the angles ∠BOC and ∠BOE, respectively, and taking half of each of those angles. Thus, the angle ∠BOC between O B ¯ ¯ and O C ¯ ¯ is double the angle θ θ p which the major principal plane makes with plane B .