Cartoon of the Hale sector boundary (thick purple lines) for odd (left panel) and even (right panel) Cycles during periods when four-sector boundary crossings would be detected at the Earth. The HSBs are the parts of the sector boundary where the magnetic polarity change, from leading-to-following (right to left in each panel), matches the magnetic polarity change of the sunspots in that hemisphere. Positive magnetic polarity is indicated by yellow, negative by blue. Based on the Cycle 20 example in Svalgaard & Wilcox (1976).