The (σ, τ) coordinates of the upper and lower points on Mohr's circle are (C, R).
The orientations that experience the largest normal stresses (i.e., the principal stresses) have no shear stress. The converse, however, is not true. The orientations that experience the largest shear stressses do not, in general, have zero normal stress. The normal stress at the orientations of maximum shear stress is equal to the center of Mohr's circle, C, where:
