A three-dimensional characteristic mode solution is developed for electromagnetic scattering by rectangular apertures in two parallel perfectly conducting planes separating three different media. The formulation is performed when the system is excited by an incident wave whose magnetic field is tangential to the conducting planes. The characteristic modes, obtained from the solution, are used to determine the fields and the radiated power with respect to the plane separations and aperture transverse location. ¿ 2001 American Geophysical Union