The objective is to use the Mohr circle to find the stress components σ σ n and τ τ n on a rotated coordinate system ( x ′ , y ′ ) , i.e., on a differently oriented plane passing through P and perpendicular to the x - y plane (Figure 4). The rotated coordinate system ( x ′ , y ′ ) makes an angle θ θ with the original coordinate system ( x , y ) .