Some general aspects of the force constant ellipses representing the multiplicity of solutions for the secular equation of second degree are studied. The concept of coupling of the two vibrational frequencies is discussed. On the basis of these considerations, one is able to derive some necessary conditions of approximations to the constants of the GVFF. It is shown that the force constant set determined by the extremal values of one of the diagonal constants, which recently was supposed to be a good approximation to the GVFF, accomplishes these requirements only in special cases.