Position of negative (blue cross) and positive (red cross) polarities on the time-averaged line-of-sight magnetogram for a low-flux active region on the left (AR 11066), a complex, high-flux region in the middle (AR 11158), and a weak short-lived active region (AR 11414) on the right at different times. The range of the grey-scale is ±100 G. The black contour indicates the region within which we search for the polarities. AR 11414 is an example of an active region that was excluded from the analysis in this paper because it is not clear which feature should be defined as the leading polarity. For a full list of excluded regions, see Appendix C.