Cartoon describing the effect of birefringence in radio-wave propagation through pulsar magnetospheres. In this representation, the reader is looking down the spin axis (circled dot). In the birefringent plasma, the X mode propagates parallel to the magnetic axis, unaffected by refraction, while the O mode follows the magnetic-field line direction (grey lines). Upon exiting the birefringent magnetospheric plasma, the two orthogonal propagation modes, the ordinary (O-mode) and extraordinary (X-mode), are beamed towards different directions. As the pulsar rotates, the two modes cross the observer’s line of sight (dotted line) at different pulse phases. The degree of mode separation depends on the frequency of the emission: at high frequencies (dashed lobes), the separation between the modes is smaller than that at low frequencies (solid lobes).