Phase position of the Crab pulsar main peak. The right axis represents the phase delay in ms derived using a radio period of 33.7 ms (averaged over the data set). The dashed line represents the position of the main peak averaged over the entire dataset. The shaded strip centred at 0.9898 marks the phase range where the main peak is measured by RXTE (Rots et al. 2004), with its width dominated by a systematic uncertainty of 0.0012.