Axes of Inertia, Principal

(or principal axes), three mutually perpendicular axes passing through any point of a rigid body that have the property that if these axes are taken as the coordinate axes, then the products of inertia of the body about these axes will be zero. If a rigid body that is fixed at a single point is rotated about an axis that is a principal axis at this point, then in the absence of external forces the body will continue to rotate about this axis as if the axis were fixed. The concept of principal axes plays an important role in the dynamics of rigid bodies.