We have established for the first time that as a result of a bilinear coupling between quasilocal harmonic modes and phonons in glasses there should be always a correlation between the position of the boson peak and the Ioffe-Regel crossover frequency for phonons. Above this frequency, phonons (i.e. usual plane waves) cease to exist. At the same frequency the density of states of the quasilocal harmonic modes, as a result of their interaction (and strong level repulsion), transforms into a universal linear function of the frequency. As a result the new harmonic modes become completely delocalized and have a diffusive nature. We show that existing experimental data are in excellent agreement with these predictions. (C) 2002 Elsevier Science B.V. All rights reserved.