Black-hole quasinormal modes (QNM) have been the subject of much recent attention, with the hope that these oscillation frequencies may shed some light on the elusive theory of quantum gravity. We compare numerical results for the QNM spectrum of the (rotating) Kerr black hole with an exact formula Reω → TBH ln 3 + Ωm, which is based on Bohr's correspondence principle. We find a close agreement between the two. Possible implications of this result to the area spectrum of quantum black holes are discussed.Gravitational waves emitted by a perturbed black hole are dominated by 'quasinormal ringing', damped oscillations with a discrete spectrum [1]. At late times, all perturbations are radiated away in a manner reminiscent of the last pure dying tones of a ringing bell [2][3][4][5]. The quasinormal mode frequencies (ringing frequencies) are the characteristic 'sound' of the black hole itself, depending on its parameters (mass, charge, and angular momentum).The free oscillations of a black hole are governed by the well-known Regge-Wheeler equation [6] in the case of a Schwarzschild black hole, and by the Teukolsky equation [7] for the (rotating) Kerr black hole. The black hole QNM correspond to solutions of the wave equations with the physical boundary conditions of purely outgoing waves at spatial infinity and purely ingoing waves crossing the event horizon [8]. Such boundary conditions single out discrete solutions ω (assuming a time dependence of the form e iωt ).The ringing frequencies are located in the complex frequency plane characterized by Imω > 0. It turns out that for a given angular harmonic index l there exist an infinite number of quasinormal modes, for n = 0, 1, 2, . . ., characterizing oscillations with decreasing relaxation times (increasing imaginary part) [9,10]. On the other hand, the real part of the frequencies approaches an asymptotic constant value.The QNM frequencies, being a signature of the blackhole spacetime are of great importance from the astrophysical point of view. They allow a direct way of identifying the spacetime parameters (especially, the mass and angular momentum of the central black hole). This has motivated a flurry of activity with the aim of computing the spectrum of oscillations (see e.g., [1] for a detailed review).Recently, the quasinormal frequencies of black holes have acquired a different importance [11][12][13][14][15][16][17] in the context of Loop Quantum Gravity, a viable approach to the quantization of General Relativity (see e.g., [18,19] and references therein). These recent studies are motivated by an earlier work of Hod [20]. Few years ago I proposed to use Bohr's correspondence principle in order to determine the value of the fundamental area unit in a quantum theory of gravity.To understand the original argument it is useful to recall that in the early development of quantum mechanics, Bohr suggested a correspondence between classical and quantum properties of the Hydrogen atom, namely that "transition frequencies at large quantum numbers should equal class...