The method presented herein, to the best of our knowledge, did not find its way to popular textbooks of elementary physics or classical mechanics, despite the fact that "It is advantageous to determine principal axes from an invariant formulation." Invariance, of course, may refer to scalar quantities other than the kinetic energy, which is the reason why we obtain the principal axes of rotation from the square of the angular momentum as well. In the most general terms, we exploit the theorem that a quadratic form ... defined on an n-dimensional real vector space and represented by a symmetric n^n matrix A such that q(A).... Ay, where y is the column vector, can also be represented by a diagonalized n^n matrix B. As outlined by the generalized procedure in the Appendix, finding the extrema of q(y) under the constraint .....= constant leads to an eigenvalue problem that diagonalizes the matrix A and results in the matrix B.