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Coherency-based approach to the problem of constructing power system dynamic equivalents has been very successful in applications. The key step in this approach is to identify groups of coherent generators. An analytic study of coherency is conducted. An algebraic characterization of coherency is given. An algorithm, based on the algebraic characterization, to identify coherent groups directly from system data is developed. A physical interpretation of the algebraic characterization of coherency is presented, where the condition for coherency is described in terms of generator inertia constants and their equivalent admittances to the bus of the disturbance.