B^^ | congruent | T^^xN^^ | = | (r'xr'')/( left bracketing bar r'xr'' right bracketing bar ), where the unit tangent vector T and unit "principal" normal vector N are defined by T^^ | congruent | (r'(s))/( left bracketing bar r'(s) right bracketing bar ) N^^ | congruent | 1/κ (dT^^)/(d s) Here, r is the radius vector, s is the arc length, τ is the torsion, and κ is the curvature.

bitangent vector | Frenet formulas | normal vector | tangent vector