Effects of IS width, IS crossing, metallicity, and rotation on the PLR. The abscissa shows δ = M_p1−M_p2, where p1 and p2 correspond to the effects as labeled: blue minus red edge, 3rd minus 2nd crossing, etc. Panels from left to right for V-band, H-band, and the Wesenheit indices W_VI and W_H,VI. Cyan solid lines show the effect of IS position (blue edge minus red edge), yellow lines show metallicity effects for Z = 0.014 minus Z = 0.006 (dashed) and Z = 0.002 (solid). Magenta solid lines show the effect of Cepheids on a 3rd minus 2nd crossing. Black solid lines show the effect of rotation for average rotation relations minus non-rotating Cepheids. Interestingly, the effects of rotation and crossing number are of similar size with opposite sign. Note also the period-dependence of all the effects, in particular of metallicity and crossing number.