This site https://en.wikipedia.org/wiki/Perpendicular_axis_theorem says: Define perpendicular axes $x$, $y$, and $z$ (which meet at origin $O$) so that the body lies in the $xy$-plane, and the $z$-axis is perpendicular to the plane of the body. Let $I_x$, $I_y$, and $I_z$ be moments of inertia about axes $x$, $y$, and $z$ respectively
The perpendicular axis theorem states $I_z = I_x + I_y $. Now if we have a rigid three dimensional body then how to choose perpendicular axis as it lies on $xy$, $yz$, and $xz$—all planes. Even I find it messy when I see the cylinder's axis to be the perpendicular or $z$-axis. I could assign axis perpendicular to it and say that's $z$. How can I determine what's the perpendicular axis?