Gengzi (庚子) and Xinchou (辛丑) Bìshàngtǔ (壁上土) refer to the earth on the wall. Although Chou (丑) is the rightful position of earth, Zi (子) is a place where water is abundant. When earth encounters a lot of water, it becomes mud, hence it is called Bìshàngtǔ (壁上土).