The terms in brackets in Eq. (8a) and (8b) are called the principal moments of inertia of a thin and rigid plate. If .... we obtain two different values for the principal moments of inertia. Generally, rotation of a rigid body in a three-dimensional space can be described most easily in the coordinate system of three mutually perpendicular principal axes with corresponding principal moments of inertia. Rotation about the principal axis with the smallest (largest) principal moment gives the minimum (maximum) kinetic energy, while the third principal axis and its principal moment result in an intermediate value of the kinetic energy.