Given two interacting atoms A and B, the cophonicity (/ k oʊ f oʊ ˈ n ɪ s ɪ t i /) of the A-B atomic pair is a measure of the overlap of the A and B contributions to a specific range of vibrational frequencies. In the field of condensed matter physics, cophonicity is a metric aimed at the parametrization of the dynamical interactions in terms of the atomic types forming the A-B pair. In connection with other electronic and structural descriptors, such as the covalency metric or the distortion mode analysis from group theory, the A-B pair cophonicity is a guide to properly select either A or B atomic species to tune specific vibrational frequencies of a given system. The cophonicity metric has been originally designed for the study of the atomic motions in transition metal dichalcogenides, but its formulation is general and can be applied to any kind of system, irrespective of the chemical composition and stoichiometry.