If the surface S₁ intersects the surface S₂ along the regular curve C, then the curvature k of C at p € C is given by k² sin² ϴ = λ²₁ + λ²₂ - 2λ₁λ₂ cos ϴ,
where λ₁ and λ₂ are the normal curvatures at p, along the tangent line to C, of S₁ and S₂, respectively, and ϴ is the angle made up by the normal vectors of S₁ and S₂ at p.