The weakly nonlinear, freely vibrating motion of a system of coupled spans of suspended overhead transmission lines is studied. It is shown that the natural vibration is the gravity mode, of which the unsteady tension component vanishes in the first harmonic. The problem was born out of a study of the phenomenon of galloping, which is a high amplitude periodic oscillation of overhead transmission lines due to steady cross wind. Particular attention is given to an intermodal resonance, which may be interesting for galloping control.