When two black holes collide, the system evolves through multiple phases. In the final phase, a new compact object forms and settles into equilibrium through the so-called ringdown, characterized by the emission of gravitational waves. The black hole’s response to a perturbation in this phase is described by exponentially damped sinusoids with discrete frequencies and damping times. These oscillations, known as quasinormal modes (QNMs), arise from black hole perturbation theory. Since the system is non-conservative, these frequencies are complex.