Different polar CHs, which dominate the CH distribution at times of low solar activity, show longer convex and concave parts at their boundaries. CATCH (Heinemann et al. 2019) was used to detect the boundaries of these CHs, whose outline is marked in each panel by red curves. The background images were obtained with the Atmospheric Imager Assembly (AIA, Boerner et al. 2012; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO, Pesnell et al. 2012) in the 19.3 nm passband.