perpendicular axis theorem

Noun

 * 1)  A theorem that states the moment of inertia of a planar lamina (i.e. 2-D body) about an axis perpendicular to the plane of the lamina is equal to the sum of the moments of inertia of the lamina about the two axes at right angles to each other, in its own plane intersecting each other at the point where the perpendicular axis passes through it.

Usage notes
Define perpendicular axes $$x$$, $$y$$, and $$z$$ (which meet at origin $$O$$) so that the body (in the form of a lamina) lies in the $$xy$$ plane, and the $$z$$ axis is perpendicular to the plane of the body. Let Ix, Iy and Iz be moments of inertia about axis x, y, z respectively. The perpendicular axis theorem states that $$I_z = I_x + I_y$$.