The mechanism of subconductor oscillation has defeated analytical formulation for a considerable period of time. Oscillation ‘inhibitors’ have been designed without a true understanding of the underlying destabilising mechanism and, in consequence, none has really proved successful. In the paper, it is shown that subconductor oscillation may be ascribed to classical flutter and that, for lines with conventional spacing (e.g. CEGB bundled lines with two or four parallel conductors spaced at 10.7 or 16 diameters), such oscillations are more likely when the vertical, natural, structural frequency exceeds its horizontal counterpart than when these frequencies are equal. The theoretical predictions of flutter boundaries are vindicated, with remarkable consistency, by dynamical model tests performed in an open-jet wind tunnel. Attention is also drawn to the possibility of static instability (divergence) or a leeward subconductor when the wind speed is sufficiently high. This possibility, again, has seemingly been overlooked in the past and warrants careful attention. It is emphasised that the use of line spacers, whose primary dynamic function is to detune horizontal and vertical natural frequencies, may, in fact, foment instability at certain bundle orientations.