Emission heights at different observing frequencies, for PSR B1133+16, based on pulse broadening (filled red circles) and component separation (open squares), and the assumption that the observed emission is coupled to the last open field-lines of the pulsar’s dipolar magnetic field. For comparison, we also show with an open circle the emission height estimate based on the delay-radius relation of Blaskiewicz, Cordes and Wasserman and our polarisation observations at 150 MHz (Table 2). Finally, the upper limit on the emission height from pulsar-timing measurements by Hassall et al. (2012), between 40 and 180 MHz, is shown with an arrow.