Residuals between I-band absolute magnitudes of SMC and LMC F Cepheids and P-L relations for each galaxy, while maintaining the slope fixed to the value obtained for the LMC. In addition, the residuals of the IS edges of both LMC and SMC, as well as their corresponding median parts, are shown as red and black triangles and empty circles. The SMC and LMC median ISs are connected with black and red lines, respectively. The vertical red line marks a pulsation period of 3 days.