Analytic investigations of interarea oscillation damping of an idealized two-machine system using active-power modulation of three- and four-terminal high-voltage dc (HVDC) transmissions are presented. It is shown that, generally, the pairing of two terminals gives the highest damping. Based on the analytical results, recommendations for how the pairing should be made are developed. Simulations, both on a two-machine system and on a more realistic 23-machine system, verify the analytical results.