The theorem of perpendicular axes states that the moment of inertia of a planar body (lamina) about an axis perpendicular to its plane is equal to the sum of its moments of inertia about two perpendicular axes concurrent with a perpendicular axis and lying in the plane of the body.
A physical body withcentre O and a point mass m, in the x–y plane at (x, y) is shown in the following figure.
