Theorem of perpendicular axis
It states that if IXX and IYY be the M.I. of plane section about the two mutually perpendicular axes meeting at a point. The M.I. IZZ which is perpendicular to plane and passing through intersection of X-X and Y-Y can be given by the relation
IZZ = IXX + IYY
IZZ called as Polar moment of inertia also.