# Orientation of mass moment of inertia

How should the orientation of the mass moment of inertia matrix be?

An example: Lets assume we have a cube and we are gonna make a joint angled 45 degrees on one of the sides. should i then calculate and orient the mass moment of inertia matrix according to this joint - angled 45 degrees. Or should i just calculate the Mass moment of inertia acoording to the initial reference frame?

Hope my question is clear.