The polar second moment of area (also referred to as "polar moment of inertia") is a measure of an object's ability to resist torsion as a function of its shape. It is one aspect of the linked through the where the planar second moment of area uses a beam's cross-sectional shape to describe its resistance to deformation (bending) when subjected to a force applied in a plane parallel to its neutral axis, the polar second moment of area uses a beam's cross-sectional shape to describe its resistance to deformation (torsion) when a moment (torque) is applied in a plane perpendicular to the beam's neutral axis. While the planar second moment of area is most often denoted by the letter, formula_5, the polar second moment of area is most often denoted by either, formula_6, or the letter, formula_7, in engineering textbooks.