The moment of inertia of a mass element depends on the coordinate system you use to define the property. It consists of six independent components: Ixx, Iyy, Izz, Ixy, Ixz, and Iyz, where the subscripts refer to the axes of the reference coordinate system.

In most cases, the moment of inertia definition requires only three diagonal terms—Ixx, Iyy, and Izz—with zero values for the off-diagonal terms. This depends on the principal coordinate system you use to define the moment of inertia and is always true if any two of the three coordinate system planes are planes of reflection symmetry for the mass element.

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